See this earlier post. Following on from that, the average length full-price eroge would probably be at least 8 hours of reading even for someone who’s 100% fluent. As the general length/amount of text in an eroge is so much larger than other comparable media, I’d argue that the story/characters/quality of text are more important than the other elements, even for an eroero game (and although it’s very much open to debate/personal opinion, on the whole I’d suggest that the quality of the writing/scenarios etc. for most eroero games isn’t as good as it is for the story-based games which dominate the top 5% of EGS rankings). At least, that’s my attempt at a possible explanation. [Edit: On the other hand, the market for low-priced eroges (4000 yen or less) is dominated by eroero games, and arguably because of the much shorter length of the stories, the other elements of the eroge become more important. Then again, these titles for the most part don’t rank very well on EGS either.]
Feel free to disagree with this post (very much personal opinion).