User blog comment:Brandon Rhea/Fandom and Curse Media are joining forces/@comment-1142365-20181214113202/@comment-26437440-20181214125657

While it's still MediaWiki at its core, it's been so heavily modified that it's really just an in-name only thing. It is still MediaWiki 1.19, but with so much stuff backported and extensions added in that it's nothing like the actual 1.19 from 2012.

It's basically impossible for them to upgrade without a huge overhaul. And given some new features don't seem to use the MediaWiki software at all (discussions, community builder) I wouldn't be surprised to see Wikia completely move away from it at some point.