I think that's really weird, but I bet that's only because I don't really have any kinks. I mean, first time is my kink (my only one, but it's HUGE) but I don't give a fart about first-time in any fandom I don't already love for its characters. Maybe it's because first time is a character-based rather than an activity based kink? If I were into bondage ... OK, I can see that I might sample a good writer's bondage story in another fandom. Yeah, it's not so weird now. I take that back.

writing one pairing in a fandom rather than another because it's more popular and brings in more readers.
Writing solely in order to get attention has never really made sense to me, but I've heard writers gripe that other writers get more readers and attention than they do - as if they're somehow owed fans and the other writers are doing something illicit, like writing "popular" pairings, in order to steal attention that these gripers think they deserve - so I know it happens. There's no reasoning with that level of neediness and self-entitlement.