The VN medium has different capabilities & expectations than an anime. Specifically, an anime needs to be cut into between 12 and 26 chunks of about 26 minutes each (i.e., episodes) of basically linear story.

While VNs have started to be produced outside of Japan (and some of them are very good), non-Japanese VNs are both cheaper & shorter on average than their Japanese counterparts. Japanese VNs are very, very long — substantially longer than the corresponding anime in terms of runtime to play all the way through. For instance, it took me 7 hours to play through Kanade’s arc in Shuffle!, while the Shuffle! anime is only 24 episodes long (for a total of nine and a half hours of watch time) — and there are four or five other girls who have relationship arcs in that game. When adapting branching VNs, rather than adapting individual branches separately, it’s normal to merge branches together & produce a hybrid — which means character development gets nerfed.

Even in cases where arcs are totally separate & the adaptation keeps them separate (such as the 007th Expansion games), an anime can’t dedicate the same amount of screen time as a VN (each arc in the Higurashi games takes about 8 hours to play through & is given 4 episodes — i.e., a little under two hours — in the anime; the first arc of Umineko is something like 15 hours in the game and is given 4 episodes while subsequent arcs are given an episode each with most of them cut entirely).

While VNs are sometimes quite long-winded, other times they use the time they have to establish atmosphere (Umineko’s oppressively slow pace is part of what makes it so impressive & memorable — ludonarrative consonance, since you see a tragedy slowly unfolding before you and there’s nothing you can do about it, just like the protagonist) or to develop characters (Higurashi is beloved because all the characters are made well-rounded and likeable through extensive development, before the paranoia sets in and people start dying) or to drop lore (Fate Stay/Night has an extensively-planned, complex, and basically internally consistent magic system that interacts in complicated ways with the society it portrays and the philosophy of life people have, and that doesn’t come through completely in the anime adaptations). Only bad VNs become better in adaptation to anime, because anime adaptation mostly means cutting (and a VN that becomes better through cutting is a VN that didn’t make good use of its players’ time).

Additionally, VNs very often are pornographic or have pornographic elements, although in the past 10–15 years this has become less common. It used to be that regardless of the quality of the story, if a VN didn’t have explicit sex scenes, it was expected that nobody or almost nobody would buy it. (Obviously, Higurashi and Umineko don’t have this problem, and Shuffle! is likewise pretty chaste, but it’s still pretty common in older games.) Upon adaptation, the sex is made less explicit or cut entirely, because it’s intended to be aired on TV. Often this doesn’t matter because the sex scenes were stuffed in as cheesecake & didn’t really matter to the plot, but other times either the sex is the only major draw (like AkihabaraStrip or Conception) or the sexual elements are integrated into the lore in important ways (like Fate, where the magic system is in some ways very sexual, in a kind of BDSM vein, such that the final arc of the Fate game had to be adapted as a series of theatrical films instead of a TV series due to all the parasitic dickworms & shadow rape and such, none of which could really be cut; after Fate’s first adaptation in 2006, subsequent installments in the franchise tried to downplay the sexual elements of the lore, but the first & best part of the franchise — the original Fate Stay/Night game — puts it front and center so they haven’t been able to remove it entirely). Obviously, if you’re removing a very important element of a game in the process of adaptation for censorship reasons, you’ll have a hard time rebalancing it & making it enjoyable.

Sometimes, VN adaptations are bad for less easily-explained reasons. For instance, the Shuffle! anime is borderline unwatchable while the game is very fun, even though the anime recreates the fairly-linear experience of the game almost shot-for-shot, with duplicated dialogue. The Shuffle! game doesn’t survive on the backs of its characters (who are largely unlikable cliches) or its plot (which is meandering, incoherent, unstructured, and unsatisfying), but mostly on its audacity: because it is dumb, it makes a point to also be big and loud (literally: the lively BGM is mixed extremely high by default, the character images are HUGE compared to other games, and the controls are tiny and minimal to make room for these character images, each of which take up more than a third of your whole screen horizontally and 90% of the vertical space). The anime is framed basically conventionally & has pretty conventional audio mixing, so because nothing is right in your face the way it is in the game, it ends up being boring. This seems to be a matter of the studio seeing the adaptation as a paycheck & not putting a lot of effort into figuring out why the original was successful (something that DEEN also did with its adaptation of the Junji Ito Collection, but DEEN’s ability to adapt is all over the place: they screwed up Fate/Stay Night but did a great job on Higurashi & Umineko considering their constraints).

While some VN adaptations are pretty good (and a reasonable substitute for playing the game, if you’re short on time), almost no VN adaptations are strictly better than the originals, basically because of time constraints. But, especially for doujinsoft games like Higurashi and Umineko, anime adaptations can often make up for what they lack in time by having much better production quality. (DEEN’s Higurashi has famously low production quality for anime, but is miles ahead of the original game’s original art, which is on par with your twelve year old cousin’s sonic the hedgehog fan art, and when the character art for Higurashi was redone, the new artist leaned heavily on DEEN’s redesigns; the same thing happened with Umineko. Both these games, however, still have backgrounds that are just public domain photographs from weird angles filtered through a blur/edge-detect/posterize filter.)

There are other concerns, especially with regard to branching. VNs are normally played to completion, with people playing through each arc in order of how much they like different characters. Often, because of this expectation, information will be revealed in some branches that influence your understanding of other branches. In VNs where the protagonist can die & is expected to die frequently, lore, plot information, character development, or thematic development can happen in abortive branches that can’t be incorporated into a linear narrative. Often, the ‘true end’ only unlocks after you’ve gotten all the easier-to-get endings, and this ‘true end’ can be much longer & rely upon information given in earlier branches. This kind of looping structure is hard to serialize without actually having loops (see the ReWrite adaptation for an example of how far wrong this can go), so hybrids can be hard to structure in a way that’s understandable in a game like this. The easiest VNs to adapt are the ones that are basically completely linear, because they’re structurally more similar to anime.