The thing with Gitlab is probably that it had to deal with the network effect/winner takes all conundrum. GitHub was first on the scene, it was the first to make a centralised hub popular and it has had the custom of many notable open source projects.

Gitlab as a product is miles ahead of GitHub at this point. (Except at the point of scaling very large repositories I gather.) There is just very little money in saas beside the bespoke enterprise contracts.

I’m not sure how revenue between GitHub and Gitlab compared before the Microsoft acquisition and now but GitHub has always leaned on being the popular tool with the network.