I think it's a lot simpler than that: for lots and lots of people, sexualized pain of whatever variety is a major kink. Not nearly as many people kink on non-sexualized despair. (There have to be some, because there is nothing in the universe that someone doesn't find a turn-on, but I suspect they're in a minority.) Why this is so I wouldn't venture to guess, but realizing it helped me be a little less grumpy about the fact that what I mean by dark fic was usually not what fandom means by darkfic.