The fascination with Richie and Eddie fanfiction, for me, almost entirely hinges on the unresolved potential between them. 'It' has this incredible, intense bond that the book touches on but the adaptations never fully commit to. So a lot of the most popular fics explore the 'what if' after they all leave Derry. You get these beautifully melancholic stories where they reconnect as adults, haunted but finally able to be honest. The 'post-canon fix-it' is massive because, well, canon is brutal. Writers give them the soft ending they deserved.
Then there's the high school era fics, which are a different flavor of pain. The sneaking around, the internalized homophobia, the fear of Bowers—it's all very potent for angst and secret pining. I've noticed a trend lately towards more mundane, slice-of-life AUs, though. Coffee shop settings, university roommates, that sort of thing. It's like after all the trauma, the fandom just wants to see them happy and bickering over domestic things. The appeal is watching their iconic dynamic play out in a world without a clown.