Agree with@jschwe, GitHub is the reason we are getting new contributors, and moving would mean setting up our own CI, which requires someone to maintain it and budget for it. I don’t see a clear reason.
One of the reasons Servo is easier to contribute to compared to other browsers is the fact that it’s on GitHub, making PRs as simple as possible. We also have GitHub issues, which are easy to access, this is how I started working on Servo, because it was easy to access and there was no gatekeeping.