Fandom is completely unusable these days without an ad blocker. I'm surprised communities still put up with it. Why hasn't another free wiki host emerged?
In short, because of SEO. Google downranks your site if it has the same content as an already existing site. This means you have to rephrase every single article on your new wiki because wikia refuses to delete anything hosted with them. So your choice is either stay with wikia or try to battle against wikias SEO.[1]