Animation in the west is largely limited by the ‘animation age ghetto’ and particular expectations surrounding what that means. Specifically, animation was generally aimed at small children & tempered by the expectation that small children cannot understand complex morality, should not be exposed to sexuality in any form, and cannot understand complicated or long-running stories. While animation aimed at older audiences has always existed in the west, it’s also always been very rare (the exceptions, films like Heavy Metal, prove the rule). This was worst in the 1980s in the US, when changes to how television was regulated produced a huge number of low-effort series intended to sell toys yet largely prevented from doing anything interesting along the way — shows that are now sort of iconic, like He-Man and Transformers.
In Japan, while television anime was largely aimed at children, it had maintained a big audience among teenagers too — in part because it was a pretty common way to adapt manga, which were (and remain) extremely popular among teens. At the same time, in the late 70s, the beginning of the Japanese economic miracle coincided with both the invention of the VCR & the coming-of-age of a generation of new animators strongly influenced by the often edgy and intellectual shojo manga of the day. This juxtaposition created the OVA market: studios had enough spare cash to cater directly to the smaller but growing fringe of adult fans of animation, and they had the technical ability to circumvent both television and theatrical release systems by shipping video tapes directly. The OVA market grew throughout the 80s, and fed a big domestic pirate community of tape-traders.
Some of the best cartoons on western TV were actually re-dubbed or re-edited japanese imports, or had had some or all of their animation outsourced to japan. (This continued to occur for the next few decades, to varying degrees: in the 90s, Cardcaptor Sakura was chopped up and rebranded as Cardcaptors, YuGiOh had lots of stuff changed to suit american audiences assumed to be younger or dumber than the japanese ones, and the same happened to Pokemon.) By the end of the 80s, a certain number of adult fans of animation in the west had figured out that these shows didn’t just come from japan but were actually better in their original format. They hooked up with the tape distribution rings there, got a taste of OVAs (which were now competing with each other for how wild they could get — a competition they lost, since Belladonna of Sadness had outdone all of them in the late 70s), and started importing — because anime was a perfect fit for folks who were outgrowing the black & white morality and single-episode sitcom-style plots of western cartoons but still appreciated what animation could provide.
Anime fandom also became a sort of attitude & identity, mostly among young teens. When I got into it a decade later, this was still the case: much like with the boom in dark & edgy comics in the late 80s and the early 90s, anime was appreciated for being more ‘adult’ than cartoons because of the sex and violence that abounded (mostly in OVAs, though japan is a little less prudish about sexual humor involving teens so even TV anime seemed pretty racy compared to the cartoons they replaced). Anime also had (and continues to have) a lot of gay representation (not because japan in general is progressive about that, but because of a combination of factors — a long tradition of dealing with sex and gender issues in a science-fictional mode starting with Tezuka but emphasized in famous 70s shojo manga associated with japan’s often-politically-repressed feminist movement, a tendency toward a style of humor that involves inverting social roles and power relationships, and a total lack of producer oversig...