Github is so ubiquitous that they can't afford to move fast and break things like Gitlab can - another team at work considered moving to Gitlab to use all these new features, found bugs everywhere and eventually went back to Github.
That said, I find the openness with which core architectural issues are discussed to be refreshing. (e.g. see the very detailed writeup explaining the root cause of the above issue, here https://gitlab.com/gitlab-com/infrastructure/issues/677)