Based on what? They rewrote the language into another that is largely backwards compatible, fixes PHP's quirks and is insanely fast.
Given the choice of migrating / transpiling an entire codebase or making the underlying interpreter / compiler and language fantastic and speedy, the latter is a big, big win.
What would rewriting the whole thing in, say, Ruby or Python have done beyond limiting the amount of "PHP sucks!" critique they so often get?
Seriously, take a look at Hack. It's very well done.
