The market segment of web hosted Git repositories was a relatively new and small market segment when GitHub got started back in 2008.

GitLab didn't appear until 2011, and at the time it didn't feel to me like a direct competitor. So GitHub had quite a bit of extra time to establish its presence and capture a large share of a growing market.

For a large chunk of GitHub's users, GitHub fulfills their needs well enough that they don't feel much motivation to make a change. Even if GitLab is better than GitHub for a person or company, it has to be better enough to be worth the pain of switching.

And I think for most users, that just isn't the case. Speaking anecdotally, there are some things about GitLab I like more than GitHub. And if I were starting out, I'd likely pick GitLab. But all of my code is on GitHub, and although I pay a monthly fee for private repos, it's small enough that it doesn't bother me.

I believe that GitHub is more popular mainly because it was first, gained traction and became synonymous for some people with git itself, and open source.