That's mostly because PHP made sense at the time that decision was made - by the time it didn't they had too much legacy to make a complete rewrite worth it. Instead, they chose solutions that wouldn't require a complete rewrite by creating HHVM[1] and Hack[2].
And remember PHP was only used for the user-facing stuff (they probably had more Java and C++ than PHP when they began moving to Hack) - if you look at what they open source[3] only about half a dozen (out of over a hundred!) projects are PHP.
