GitHub does have that popularity and mind-share going for it, so I can see why people might not care to leave and create a new account at GitLab. The primary concern is the license which could be identical at either platform so that's a non-issue.

The primary difference is that GitLab (referring to CE, the 'Community Edition') is completely portable and self-hostable. That means if things change later or the platform host gets a legal challenge or takedown request, you are free to say 'go pound sand' and take your data and platform elsewhere. You are not at the mercy of the platform provider.

For a classic example of a community effort where the rules were changed after a great many users altruistically gave of their time and effort see CDDB, also known as Gracenote[0]. And as I typed my original comment, this thread[1] was on the HN front page with some choice comments illustrating the difficulty of doing a 100% clean export of your data from GitHub.

Google has a great initiative, humorously called the 'Data Liberation Front'[2] which makes sure users can easily export all their data in a useful format if they want to take it elsewhere. I used it when I closed my Google accounts and I can report it works perfectly. I see no comparable effort by GitHub. Users like me check for this early and it's absence is always a red flag.

Hope that answers your question and if my assumption that you don't want to use GitLab because of the friction of creating yet another online account is incorrect please clarify what your real reason is.

[0]https://en.wikipedia.org/wiki/CDDB

[1]https://news.ycombinator.com/item?id=11054011

[2]https://en.wikipedia.org/wiki/Google_Data_Liberation_Front