The Terraria wiki is hosted on wiki.gg, which the Minecraft wiki editors strongly considered (reading through the discussion, it seemed like a strong second choice behind Weird Gloop/RuneScape wiki). But wiki.gg really has a lot of the same problems as fandom, just to a lesser degree. Still has a fair amount of ads, still has wiki.gg branding on the website, and most importantly in the discussions I read, they wouldn’t be able to use a domain the Minecraft wiki owns (as in it wold have been at minecraft.wiki.gg rather than minecraft.wiki). The big problem with not having their own domain is that if things ever go south with wiki.gg (for example if they get bought by Fandom), they would be starting from scratch as far as SEO/discoverability goes, same as they are now.

Weird Gloop (the host for the RuneScape wikis) offered fewer ads, a comparable hosting infrastructure, and the ability to use their own domain, as well as a lot of experience forking from fandom successfully, which sounds like it was really valuable to them.