I understand that people would be annoyed by a forced switch, github has a working issue tracker, wiki, and not to mention the awesome github pages. But, I just think loosing them is not that dramatic.
But when people say that GitHub has better UI, they're more talking about the clean menus and intuitive UI. Some of it might just be getting used to one design over another. Although, BitBucket to me has always felt too cluttered.
The opening/home page of Github is still better though. What really got people into Gitlab was the self hosting. Github has depended on selling their enterprise version. I was at a talk where Wanstrath said something to the effects of their expensive enterprise version that only people with money to spend need. .. (Years after I left my job at a state university ... they bought a license. -_- I hated how they paid for a lot of stuff they didn't need).
Even though Gitlab may lack in some UI elements, it's more than good enough and it doesn't hinder work. I'm at a shop that still uses the community edition too.
Gitlab and Bitbucket really cut into Github's model. There are more clones out there now too. If you really want a self-hosted Github like UI, there's Gogs too.