This is a problem many startups have. Because people use brands as a shortcut to determine whether something is of sufficient quality to be worth taking the time to investigate, being a new unknown is a bit of a hurdle to overcome.
What should I use jQuery and not some other .js library? Because I know jQuery will still be there in a year.
i would guess that the majority of them wouldn't even know to look for phpc or any alternative. there just hasn't been any news of something like HPHP before HPHP. if you want to run php, you go to php.net or install it from a package and go from there.
combine the above with very little actual need, very little public information on how to build a large site, and lack of maintainer interest and phpc just won't get major traction.
I would have loved to have found phpc several years ago when i was still working on a large site that had performance problems.
we're talking about websites that are in the top 5% of all sites (by traffic) that would even need to think about using something like phpc or hphp. the number of those that would be willing to use a freelancer to handle their scaling issues are probably in the single digits at best.