i haven't read fics outside the kpop fandom but i do agree with you saying there are very few longfics for majority of the fandoms i've read. the only fandom i've realized that has longfics is exo. most fics are 10k+ and many "one-shots" are even like 50k so they're super long O_O I've seen many chapter fics going up to like 30 chapters too ;o

i guess a factor with the production of long fics is the age of the fandom? fandoms with more older people (20+?) usually has really long fics for some reason. the authors i know who write extremely long fics are all above 20 years old and it's probably because they read novels and etc as a kid so i guess they have more experience with longer works? idk, maybe i'm generalizing too much but yeah LOL

(excuse my randomness... idk what i'm saying orz)