The simple reason is that there is no guarantee that there will ever be a Fedora-hosted Gitlab and if so, it likely will not happen in the next year or two. Someone would have to make it their priority to provide that and then gather the resources within Fedora to create this, which means going into existing structures, rallying those people that really want it, and then get to the task of implementing that solution. These things take time and as far as I am aware, no-one is currently on this task nor really wants to be. Or is it already in progress?
Let’s say someone did it and it’s now early 2021. Whether we’re on GitHub or Gitlab, in that time we have grown more awareness and have a mature process as well as possibly have gotten actual resources dedicated to Silverblue, depending on growth. On GitHub, we have the CoreOS community already there and the CoreOS developers which helps with visibility and possibly fixing things faster as rpm-ostree is also there. The growth factor is also the biggest there. On Gitlab, we half-satisfy the people who are against the fact that GitHub is proprietary and it is a good platform which gives us all the UX and features we need to simplify our tasks. Personally, I really like and prefer Gitlab but from a strategic point of view, GitHub seems better to me. There is also a chance of Microsoft open sourcing the platform, weirder things have happened.
So the goal is growth and resources. The community members can decide where they would like to contribute, there is plenty of information in this thread alone.