In my experience, about the same as yours almost ten years, the only other fandom I've seen with the number of quality fics is The Walking Dead fandom. (My personal jam is the Desus fics). Criminal Minds has a few great ones, like blueoleandar93's fics. It might have to do with how some fandoms prioritize their fandom activities as well. I've seen fandoms that don't place a lot of importance in fic writing and reading.

well, yeah i guess that also makes sense–i guess the 1d fandom is a fandom that puts reading and writing fic on a pedestal, you can see it from the sheer number of ao3 fics and the wattpad fics