I think the open source requirement is holding them back, people also want to start projects without immediately making them open source. Maybe they don't know it yet.
These people, if they want to avoid github, will host their own private gitlab instance, or use just a local git repo, or another private repo, until they decide to make it public.
I think this makes a big contribution the disparity.
To add, I think it's fine, as long as anyone can clone the github. It's not like I have any solution to this.