No software is a panacea. Decisions are made which favor some use cases over others. When you are as large as google or facebook or github, those decisions wh

No software is a panacea. Decisions are made which favor some use cases over others. When you are as large as google or facebook or github, those decisions which don't naturally mesh with your workflow become more painful over time. And when you have enough in-house resources, the cost to working with the existing solution far exceeds that of extending it on your own or building your own. Hiphop is a good example where Facebook saw certain things they wanted to optimize in PHP, and it made far more sense for them to work on their own thing (many of those decisions underlying hiphop make sense for facebook but not for most companies deploying PHP)