It's because of a few things:
1) There are less fics overall
2) Of the fics that there are, fewer of them are written just for the hell of it, or on a dare, etc. People actually put some thought and effort into it, which makes them better, therefore people reply, making the author happy, so that they continue to post with it.
3) There are more readers than writers here nowadays, from what I've seen.
[Just on a side note: Every fic I've ever written has been abandoned. Back when my first fic died, I was full of ideas for new fics, I must've posted 5-10 of them over a year or so, and not one of them got a single reply. Not a one. THAT is why, I feel, that a lot of fics get abandoned. Authors get discouraged, because nobody bothers to reply to what could be a promising fic.]