On the other hand, Wikipedia is going to have to migrate back off of HHVM now that Facebook is planning to break compatibility with PHP and focus totally on Hack [0]. So hopping on board someone else's solution can bite you if their priorities change.
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.
