Another reason for the lack of Western Cartoons fanfiction on AO3 could be because there is still a lot of fanfiction on FFN? In fact, have you ever thought about doing an analysis to compare the number of fanfictions from the same fandoms on both sites? And maybe also compare the anime/manga category
Yeah, good thinking... I believe historically a lot of animated shows had most of their fanfic on FFN, though in more recent years at least some of those fandoms have gotten very popular on AO3, and FFN growth has been tapering. I've done a number of comparisons among different fandom archives and platforms, but I haven't compared the sizes of a bunch of specific fandoms on on AO3 vs. FFN - has anyone else? Somewhat related -- I compared the biggest fandoms on AO3 vs. FFN in 2015... lots has changed since then, though.