My fandom, reading/writing fanfic experiences comes from kpop only. I've been in fandom for a good amount of years (almost 8 years and counting) and my safe bet is that is a miss of how old the band is, how old the actual fandom is, the age of the fans and how set is the fandom canon. Im still reading a Shinhwa fanfic from a couple of years ago (it has three long chaptered fics and is not finish) and DBSK fandom has a great amount of long fics. I remember it took me two weeks to read a fanfic that it wasn't finished (as of this date it is... well sort of LOL). I've read long fics in Suju fandom and some of Shinee fandom. Canon in fandoms do help with the amount of fics and the length of it. With the separation of DBSK & JYJ the Yunjae fandom hasn't suffer at all. They keep spitting all length fics in all the possible genre, is weirdly fascinating.
I think it will take time for younger bands fandom to get to that point or if older fans fall in love with the younger groups maybe good quality chaptered fics will come up.