The people who initially developed HPHP (the PHP-to-C++ transpiler) were not the same people who initially developed HHVM (the VM/JIT). I was one of the people who started the HHVM project, and we recognized early on that we must base our work on HPHP to have any hope of eventually replacing HPHP on Facebook's production systems. Indeed, had we chosen otherwise, our project would certainly have failed. It took us three years to initially ship HHVM, and HPHP's ancillary ecosystem dramatically diverged from PHP during that time. Furthermore, the people who initially developed Hack were yet another team, and they based their work on HHVM for much the same reason as HHVM was based on HPHP.

In an ideal world of perfect cooperation, perhaps all these projects would have been enhancements to PHP, but a chain of independent rational decisions produced very different results.