I assume that Wikimedia works on a rather tight budget. Choosing (and unifying on) tech stacks with a larger supply in devs seems to be an economically reasonable choice.
The other side to using PHP was having support in other host providers. Wikipedia is not the only installation of MediaWiki and there has been consideration in the past for those installing MediaWiki on shared hosts where you don't necessarily have root access to install things like node. Moving forward that's less of a concern because you can containerise MediaWiki (and the other services), but not even Wikimedia run that in production yet AFAIK.
However, even if they weren't budget constrained (which they aren't) unifying on a single language used by the majority of their devs isn't a bad idea, especially when the effort to port the entire stack to a new language would be unjustifiable.
server-side JS was a thing 10 years ago, but it didn't offer enough benefits to switch. same with python, java, ruby - all existed, but didn't offer enough benefits to switch then, and probably still don't now.
also, what would be a "larger supply"? C? Java? C#? JS? PHP has a huge supply of developers at all skill levels, which may make it just as easy (or easier) in finding the talent they need. And... hey - they wrote that initial parsoid in JS and... they've doubled the speed by converging on PHP.