From the Flarum README:
As it stands, there is a need for modern, well-architected, powerful forum software that is easy to use and self-host.
PHP remains the most user-friendly language for deploying web scripts, especially on shared hosting.
Same reason we use PHP instead of any other language: because it's widely available. Vagrant won't be the primary method of installation — the aim is to make it drop-and-run on any standard shared host. This is an app for users, not developers.
No point in writing in Hack and converting to PHP if our primary target is PHP.
As for performance, PHP 7 will bring huge improvements.