I don't technical limitations are really the reason (I'm a GitLab fan and have observed GitLab's product accelerating much faster than GitHub's). It's just that the community has been established on GitHub for so many years.
I don't think it has something to do with the community. But rather that GL and BB are losing to GH in terms of UI / UX. Well, at least I wanted to switch to GL long ago, but couldn't due to unsatisfaction by their user interface.
They have been changing the UI quite a bit and GL has changed significantly feature wise too in the past 12 months. It might be worth another look.