I think it's because Reddit has a strong grip in the NSFW communities (but ironically it bans people without reason - happened to us, and as stupid as it was, we'll try going back maybe next year because the community is just bigger -, and bans subreddits everyday), and there's really not a competitor. I mean, there's not even a massive NSFW-focused (not paywalled) social network. I've read about FetLife but it seems to be too kinky-focused so OC mild and non-BDSM content would probably be not too welcome there. And then there's a lot of paywalled communities that don't have too much of an audience. At least there's BlueSky, but it doesn't have the convenience of communities/subreddits. Hope Lemmy grows now that Reddit seems to go faster to its own destruction