S&S fandom seem to be a lot more inclined to comment than some of the others; in the top ten by comment, four are S&S fics.

I think with Knightmare, everyone was already used to posting their fics on forums and personal websites, and didn't see the need to move to AO3 when that came along. Whereas I approached from the other direction; I was already used to posting on AO3 when I dipped into the KM fandom.