For years, HHVM had a significant edge in performance and a significant handicap in compatibility. Today, PHP7.1+ is ~at parity in performance but the community still has not fully embraced HHVM. In fact, some flagship projects like Symfony even stopped on purpose. Increasingly, the main reason remaining for running PHP on HHVM is if you really want static typing - and this is not a natural preference for most PHP teams.
Hack/HHVM is breaking compatibility with PHP. MediaWiki is a large PHP software project. Other MediaWiki users run the software in non-HHVM environments.