I'm not super well informed about this but if I remember it's because FB would have had to play nice with the community, make RFCs or whatever it is they use, discuss with the community and then the biggest hurdle: vote.

by making hhvm, it allowed them to move at break-neck speeds with absolutely no reason to ask for permission.

The social aspect of it probably turned them off at a corporate level, if that makes sense. Where as throwing more hackers at the problem was totally in their wheel house. IMO.