People are worried about a company with a bad track record in open source to control a vital piece of infrastructure. Lately Microsoft has improved a lot, but people might still have “evil behaviour” such as during the browser wars, their “Get The Facts” campaign or license fee agreements with Android manufacturers in mind.
Personally, I’m not so worried (I really like some of the work they are doing related to Kubernetes, e.g. helm, brigade and draft, which are all hosted on GitHub), but I understand the worries of older developers.
Also it should be noted that GitLab has some benefits over GitHub, one being that GitLab’s community edition is open source, while GitHub as a platform is closed source. Also GitLab seems to be a better solution for managing large scale centralized open source projects such as GNOME (https://about.gitlab.com/2018/05/31/welcome-gnome-to-gitlab/). Even if GitLab itself gets acquired at some point, the platform could be forked and continued independently. Another major benefit of GitLab is that it can be self-hosted.
For now Microsoft has announced that they will run GitHub as an independent company, which should also remove some worries. Also, the new CEO of GitHub, Nat Friedman, was - ironically - involved with a company developing major parts of GNOME early on, such as the Evolution mail client.