Well an obvious reason is all the PHP site that have already been written, are crucial components in many businesses, and cannot be upgraded to Hack. Those sites' authors may still want access to all that good stuff.
In fact, this kind of feature may make it easier to convert those applications to Hack in the future.
You can freely mix Hack code and PHP code on the HHVM, calling functions and classes defined in any PHP file inside you Hack app. Then you can slowly evolve the codebase to contain more and more Hack and less and less PHP.
That's the huge point that nobody seems to see.
But then again, considering how many other huge points PHP developers seem to miss in general, maybe it's better to just let Node.js eat the world :)
In fact, this kind of feature may make it easier to convert those applications to Hack in the future.