It's not just a matter of operational cost. We spend a lot of time and effort making our code accessible to new contributors, and lowering that barrier is very

It's not just a matter of operational cost. We spend a lot of time and effort making our code accessible to new contributors, and lowering that barrier is very important. Contributing to code in Mercurial is hard (but getting easier with Phabricator), whereas code in GitHub is easily accessible. There is also the question of integrating with external services. GitHub has a very mature ecosystem that's helpful to developers and lets us ship better products faster.