HipHop was a cross-compiler which facebook abandoned for HHVM, a rewrite of the PHP engine with faster internals. It always had compatibility issues, but the improved performance made it interesting (afaik nobody outside facebook really got that much into Hack the language). PHP7 was a rewrite of the internal data structures which put it on close enough performance footing. Because HHVM was never a drop-in replacement for PHP and moved further away over time the community ended up sticking with PHP7 and now pretty much only facebook uses HHVM, with the big players that moved to HHVM all transitioning back to PHP7.