First off, communism as in a stateless, classless, moneyless society has never been established.

Secondly these “socialist" experiments mostly consisted of variants of Marxist-Leninism which was Stalin's view of Lenin's interpretation of Marxist doctrine. But it isn't the only one and certainly not the definitive version of communism. To associate other leftist thought like Libertarian socialists with the atrocities committed by ML states like the USSR is an expression of utter intellectual bankruptcy.

I myself identify as a communist, albeit it of a significantly different brand and one you won't find commonly on leftist circles. But that doesn’t mean I support the Soviet Union.

But I could give you my reasons as to why many of these Marxist-Leninist experiment's of the 20th century failed, I'd be more than happy to oblige, and explain why state socialism failed.

• Reliance on Heavy Industry

Many of the socialist countries that came about in the 20th century originated from primarily authoritarian agrarian societies with large classes of peasants and only a small class of urban workers, like Russia and China or set up for the sole purpose of resource extraction for their colonial overlords like Cuba. So they were woefully behind the times productively and technologically, which left them vulnerable to internal dissent and attack from stronger international counter-revolutionary forces. Hence, why many of these countries committed to 5-year-plans that were essentially government centrally-planned and coordinated efforts to boost productivity, build up heavy industry, and get to a productive standard to rival the foreign nations. In fact, the primary reason as to why Stalin initiated his 5-year-plans was to rapidly develop the Soviet Union’s economy to make it more self-sufficient and productive to match the capitalist west, for he believed that conflict with the capitalist western nations was inevitable and he had to prepare the country for it.

(which tbf wasn’t far-off considering the Nazi invasion and attempted encirclement policies enacted by the USA to contain the USSR after WW2)

You can see this being reflected in many of Stalin’s speeches and personal writings.

In the short term, these plans did prove effectively as industrial output grew exponentially rivaling that of the west’s, more people were leaving the countryside to find employment in the cities, life expectancy grew, literary rates increased, and boost productivity. However, as a result of these regimes rapid push for industrialization, they neglected other vital sectors of their economy, like agriculture or consumer manufacturing. Now there were ML states like Castro’s Cuba and Thomas Sankara’s Burkina Faso, where they did encourage diversification of the economy to boost economic productivity and the livelihoods of the citizenry. But overall, most of these ML regimes depended upon heavy industry and hardly diversified to focus on other sectors. Hence, around the turn of the 60’s decade, as the world begun to transition from an industrial manufacturing-based economy to a post-industrial service-based economy, these ML regimes struggled to effectively make that transition. So it lead to a stagnating economy with declining industrial outputs, reduced productivity, and less innovation. Gorbachev attempted to remedy this with his reforms, but at that point, it was too late and the Soviet Union(along with most of the socialist countries) was already on the brink of collapse. Only a handful of socialist states remained in the fallout, which either resorted to heavy revisionism, becoming the very capitalist nations they despised like China or Vietnam or becoming isolated backwards hermit kingdoms like North Korea with the façade of socialism.

• Development of a siege mentality

It is undeniable that there were countries that wanted to destroy the USSR and their allied ML states, the cold war was literally an ideological conflict between the capitalist west and socialist east over who would reign supreme on the international scene. But in truth, the USSR and their allies were under constant threat and encircled by rival powers. So these states had to devote a large portion of their social spending on the military to keep up with the west and boost their defenses in case of an attack.

However, as a result of this wasteful military expenditure, overtime social funding into providing vital services like healthcare or focusing on boosting the nations economic efficiency begun to dwindle and the quality of these services and jobs deteriorated, leading to a growing resentment and disconnect between the working classes and the heads of state. The USSR and most of their allied or satellite states failed to properly maintain the organic relationship between the base and the superstructure, allowing it to decay and become susceptible to counter-revolutionary ideas and movements.

Until finally when the USSR and their allies came crashing down, hardly any members of the proletariat came to the defense of their states, hell, they even participated in their downfall.

• Vanguardism

One of the core tenets of Marxist-Leninism is the belief that in order to properly facilitate the transition between capitalism to communism, the worker’s state would be guided by a worker’s party consisting of the most advanced members of the proletariat, who’d commit themselves to preserve the worker’s state in the face of counter-revolutionary and reactionary backlash and efficiently manage their centrally planned economy.

This worked in the short term, but overtime this reliance on a vanguard party diminished the power of the proletariat and deprived them from owning the means of production, as instead those were controlled by state party officials and bureaucrats who had to make quotas to fulfill those plans. This created a new bourgeoise class that exploited and profited off the backs of the labor of the proletariat to strength the state and line their own pockets. Becoming what many call State capitalist. The Bolsheviks tried remedying this with a frequent rotation of the cadres to ensure corruption would be mitigated, but this proved ineffective, as members started caring more about inner party politics, political favoritism, and personal enrichment over the well-being of the proletariat. Stalin’s fatal flaw(among many) was that he failed to adequately transition the USSR to persist past him and effectively no longer solely rely on him or a vanguard to propel the revolution forward.

So when Stalin died, the old Bolshevik guard solidified their power-base by removing the rotation of the cadres, keeping incompetent and corrupt politicians in places of government management and state bureaucracy. They abandoned any pretense towards the cause of socialism entirely in favor of rising up the party hierarchy and conform to its agenda to gain more at the expense of the many. This is the logical conclusion of Vanguardism and Democratic centralism, despite whatever good intentions initially may of existed, when the workers party was established. Overtime, the vanguard party becomes an inefficient and bloated bureaucratic entity that oppresses the working classes and enriches themselves. Becoming bourgeoisie in all but name.

Anyone who didn’t follow the party line and conform were routinely purged from office or outright slaughtered if they’re perceived as a significant threat to their consolidation of power. As we’ve seen when the Soviet Union crushed the Anarchists in Ukraine and when Communists in Spain crushed the Anarchists in Catalonia during the Russian and Spanish civil wars respectively, because they saw these movements as counter-revolutionary and a threat to class solidarity, thus their consolidation of power within the revolution.

But I think the great Marxist thinker(and my personal idol) Rosa Luxembourg said it best:

“Freedom only for the supporters of the government, only for the members of one party – however numerous they may be – is no freedom at all. Freedom is always and exclusively freedom for the one who thinks differently. Not because of any fanatical concept of ‘justice’ but because all that is instructive, wholesome and purifying in political freedom depends on this essential characteristic, and its effectiveness vanishes when ‘freedom’ becomes a special privilege”-Rosa Luxembourg, The Russian Revolution: Chapter 6.

Now, I personally believe in the necessity of a transitionary state, but I refuse to believe that a vanguard worker’s party is necessary to run that state and propel the revolution forwards. I believe we should create a genuine worker’s state, where all members of the population do not affiliate with party doctrine, but unite through common interest, universal enfranchisement, and class solidarity. To establish a dual balance of power between the workers and the state, to keep the state from succumbing to corruption and decadence and remain subservient to the interests of the proletariat, whilst effectively organizing resistance against counter-revolutionary and reactionary forces.

Those who disagree with state policy, would be allowed to issue a petition of grievances to the government and allow for open debate on the floor of the communal assembly and worker’s council, until an effective satisfactory compromise is reached between the two sides, and the state commits to it through extensive planning and democratic mandating. Which would be organized and overseen by a bunch of experts in respective fields(such as science, industry, economics, etc.) that would manage the projects for a set number of times(5 years), before being rotated out to allow the young new guard to take over.

I believe this system of governance I devised would be better and more efficient at combating rival capitalist states, whilst resisting the temptations that brought down the vanguard parties and keep on pursuing the cause of international socialism and class struggle. A happy medium. A true worker’s democracy.

• The establishment of an inorganic relationship between the base and the superstructure and An Ideological schism within the socialist cause.

In Marxist terminology, as I’ve described above, the base is the means of production and it’s relation to it, influences and shapes the superstructure which comprises all the non-economic aspects of a society, like culture, art, philosophy, ideology, etc. to justify and maintain it’s existence. They rely on each other, shaping and maintaining the other.

Societies across history have risen and fallen based on how they form and maintained the fragile balance between these two forces to accommodate the needs of society. However, given certain conditions that relationship decayed overtime until it collapsed. But sometimes, like in the case of Eastern Europe after World War 2, the USSR forcibly imposed its will and might onto the people’s of Eastern Europe. Establishing socialist governments in occupied countries often against the will of the people and backed up by military force and state coercion, which made these regimes universally disliked or distrusted by their populations. As these regimes served as mere buffer states between the USSR and it’s capitalist rivals to the west, exploiting the resources and vulnerability of their satellite states to prop itself up and maintaining order. Utilizing blunt military force to keep its tedious and fragile “alliance” together, like when Soviet troops crushed civilian dissent in East Germany(1953), Hungary(1956), Czechoslovakia(1968), Poland(1980–81), and Afghanistan(1979–89).

Forcing these Eastern Bloc nations to pay for war reparations as compensation for the damages the USSR sustained during the Second World War, which put a tremendous strain on these nation’s economies, especially East Germany, which set back their development.

These regimes hardly managed to justify themselves through an organic relationship between the state and the population, as their power was mainly kept through force, which alienated the working classes. Even other Marxist-Leninist figures like Mao routinely criticized the USSR for its role in what he called “social imperialism”, by imposing its will onto weaker nations. He also argued that the Soviet Union's collectivization of agriculture by means of state expropriation represented a "rightist deviation" by substituting the action of the state for the grass-roots action of the peasant masses. Betraying the socialist cause.

This ideological divide would fracture relations between the USSR and PRC, leading to heated tensions and a split in the socialist cause, as states choose which state to back in its struggle to claim leadership over the international class struggle which the west happily exploited to undermine the USSR and help win the Cold War.

Conclusion

The main reasons I believe the Marxist-Leninist states of the 20th century fell are due to these five major factors, as I will summarize here:

1. Reliance on heavy industry which limited economic growth, stifled innovation, and lead to stagnate levels of productivity and GDP growth.
2. Threats on all sides lead to these states developing “siege mentalities” making them waste precious money and resources to combat outside aggressors that could’ve been utilized better elsewhere
3. Reliance on a vanguard party to solely manage the functioning of the state, whilst almost completely sidelining the will of the masses lead to a bloated and inefficient bureaucracy, staffed by incompetent and corrupt party officials, that became a parasitic social class that reaped the rewards brought upon by the labor of their workers, forming a new class of State Bourgeoisie.
4. The establishment of an inorganic relationship between the base and superstructure, that was purely maintained through blunt military force and state coercion leading to great alienation amongst the people, making it more susceptible for counter-revolutionary ideas and movements to seep in and destroy them
5. The ideological divide between Mao’s china and the USSR created an irreparable rift within the socialist sphere, leading to a broken and disoriented lack of coordination between competing socialist states over who would be the leaders of the international class struggle, which the capitalist west exploited to help them win the Cold war

Marxist-Leninism is a failed ideology and has proven that without rigid balances of power and proper accountability from the proletariat, then it will be doomed to collapse in on itself and succumb to the worst impulses of the state. It’s time that leftists move onto something new that could prove more efficient and effective at rallying the masses to establish a proper worker’s state and propel the cause of international socialist successfully to overwhelm and defeat the global capitalist world order.

(Disclaimer: This is not a hit piece on any of my Marxist-Leninist followers or posters btw, I respect and like many of the ML quoran people I’ve meet here on Quora, I just have a lot of ideological disagreements with them over how to achieve the revolution. Please be constructive and thoughtful when responding to me, if you’re being needlessly crude, rude, and douchy, I will delete your comment and block you, just a forewarning)

I hope this post helps! Have a wonderful day comrades!

Footnotes