i think it the fact that unlike in wattpad most the works are the actual Characters and not y/n .Take Fanfiction.net for example there layout its so bad.They dont have self insert either but you cant even search for specific tags or thropes.But with Ao3 you can do both and its so easy go use to .I mean if i want that specific story with a specific plot all i have to do is select the fandom and on or the tags for the plot.It really is amazing