GitHub is popular because it's popular. A lot of it is network effects. As far as I can remember, they were the first user-friendly git web host. (Key being user-friendly).

GitHub has also been really great at giving back to the community (in ways that even GitLab has benefited I'm sure).

But I doubt the source of their popularity is the integration, although their robust APIs have made the ecosystem around them flourish.

The integrations are specifically why I use Github. When I had been on Bitbucket projects a few years ago, I remember how deficient they were with integrations. It was painful using Bitbucket for that specific reason.

well they also did some cool stuff like electron and github desktop

It's true that they also make widgets, but the proportion of people who use GitHub because of their desktop client has to be vanishingly tiny. GitHub became popular originally because of pull requests. It stays popular because of network effects that counterbalance its deficiencies (terrible search, weak bug management, short analytics window, etc).

With GitLab, the convenience is just another option, though in my experience it's a very powerful option that will get you further, easier, than many of the alternatives. There's nobody saying you can't run your CI on Jenkins or Travis, or deploy to whatever, wherever. GitLab gives you some powerful options built-in, whereas GitHub _requires_ you to bring your own for many of those use cases.

Not to mention it's open source, so if you find something you need, you can actually contribute it to GitLab. Not that it won't be work for you, but it's an option beyond asking somebody to put it on their dev team's roadmap.

It is not open source. That’s a bullshit assertion. It’s open “core.” There is a difference. And, you can write custom integrations for Github.