Perhaps they need a bit more control over the project? PHPs voting system is a bit weird, in the past a lot of great proposals had been voted against. If there was no for a few individuals investing a lot of personal time and energy to push for votes many of the features that are now highly praised in PHP7 wouldn't even be there. I can only presume that this is not a very good solution for Facebook, as they probably need a way to have a better control over the roadmap...