I use all three, and I still like GitHub. It works well, and it's incredibly well supported in the wider ecosystem of services - every CI product, bug tracker, comms product and analytics tool you can think of integrates with Github. Plus, for most large-ish projects, the web UI isn't where anyone spends much time so it doesn't really matter.

I'm intimately familiar with GitHub's UI from both my OSS work and previous employers. I know GitLab can do everything GitHub can do (and more!) but it infuriates me how long it takes me to figure out how to do things I can do in my sleep on GitHub's web UI.

I don't think it's just a familiarity thing; I've been using it for months and still haven't adjusted. Gitlab's UI/UX has gotten better by leaps and bounds, and I prefer it to BitBucket, but it still feels like it has a ways to go.

Starting out with GitLab and then moving back to GitHub, I feel the exact same, but in reverse, heh, so you're not alone

Hah! Utterly delightful. So much for thinking "oh, it absolutely can't be a matter of familiarity"

Well, because it's cheap, I have literally no issues or problems with it, it has all the features I need, integrates with everything, and is the most popular repository of open source code.

Why move unless there's a compelling reason to?

I dont think so, Github has way better integration with host of products and Nicer UI than either of the two you mentioned.

When I hear "Github has better integration" I think "You need better integration because half of Gitlab's features are provided by third parties on Github".

I don't care about the integrations on Gitlab because it already has everything I need, plus they ship new features at a rate of more than one per decade.

Well that or Unix philosophy of do one thing and do it best.

I use GitLab exclusively these days. GitHub annoys me because I can't turn off PRs for mirror repositories, so I end up either having to sync or to manually tell people to take their PR and redo it on GitLab.

"No one ever got fired for buying IBM."

I feel like Atlassian is IBM in this scenario. Bitbucket, JIRA and Confluence isn't everyone's favorite but you know they get the job done.

Of course, IBM has Rational Team Concert. It did... everything. But even teams within IBM were switching to GitHub Enterprise last I was there.

Network effects really, that and name familiarity possibly?

Mostly because of the "hub" part. It used to be the place where everyone else was. Up until the Microsoft buyout when people started jumping ship in droves.