As a fansubber who is also a bit involved with fan translation, here is what I personally feel on this matter:
1. As Clephas and others rightly mention, a VN is much, much longer in terms of raw line count than your average anime. As an example, the DCIII anime had 3702 lines of dialogue in 13 episodes, whereas the trial alone for Fureraba has 5011 lines in the trial, with approx. 48000 lines in the full VN. This also means a lot more commitment than many anime. I feel that quite a few people are unwilling to commit to such a task.
2. The average length of a line is often more in a VN than an average line in an anime series. Usually, in anime, it's mostly a sentence at most. But for a VN, taking an example (from Irotoridori no sekai):
相手が自主的に話そうとしないことを、根掘り葉掘りと聞き出そうとするのは……何というか、綺麗じゃない。そんな気がするからだ。
(If someone is unwilling to speak out about the circumstances by themselves, interrogating them thoroughly about that is... how do I put it, not very elegant. That's how I felt.)
That is one "line". It's quite longer than your anime dialogue, and this is what struck me as most different when I started working on a VN.
3. This, while I find hard to digest, but is true, is the fact that anime seems to have a larger fanbase than VNs. Thus, it maybe so that working on a VN gives lesser recognition than say, speedsubbing a popular anime show and releasing it in 6-12 hrs after airing.
This same reason holds for why people don't fansub older anime to quite an extent, in my opinion.
4. Another point would be that many people who like anime, cannot or do not find VNs very enticing themselves, and as such are not much enthusiastic about working on them. It's the variety of human nature. This should include fansubbers to an extent too. My best friend used to be an expert Translator of anime, but he didn't like even reading VNs, much less than working on one. When I asked him why, he said he preferred more of a "motion picture".
But hey, things might be changing, even if bit by bit and very slowly. I do hope people and especially fansubbers take more interest in VNs in future.