The immediate reason was the whole "PLAGURISM!" kerfuffle - Cassandra Claire's work (which was immensely popular within the HP fandom) was booted off of it for having a few too many Buffy quotes. Discontent about FF.net had been growing for a while, though - since they let anyone upload with no moderation, many of the fics there were low quality or unfinished, many of the reviews were one-liners or abusive, etc.

Thank you for the answer. It seems that it dates back from before I started reading fanfiction, so I wasn't even there then.

It's true that sorting through the low quality fics on ff.net is complicated. Though I appreciate being able to leave a positive one-liner as a review (when it's negative, I usually try to elaborate a little more).