Nothing except their own arrogance. See svnhub.com. It was registered by github to block potential competition long before SVN support on github became available.

Preventing some other party from riding on the coattails of your trademark is reasonable behavior as far as I'm concerned. This doesn't prevent anyone from offering an SVN host, just from stealing free publicity from GitHub in the process. That isn't arrogant at all.

Then they shouldn't have been riding the git hype wave and using/stealing that trademark.

git is not trademarked.

git is a trademark. Here's the USPTO trademark registration info: https://tsdr.uspto.gov/#caseNumber=85961336

I've tried that link on three browsers (Safari, Firefox, Brave) and there's no contents at any of them.

Nevertheless, I'll assume you're not playing some weird game, and that git is trademarked, in which case I stand corrected. It's therefore safe to assume that GitHub and GitLab are allowed to use git in their trade names, according to the holder of the trademark. Unlike a notional "SVNHub", the trademark violation in those two names is quite clear, so they either have explicit permission and pay a license, or Linus (presumably) is ok with their existence.

I'm sorry that the link seems to have expired. It would show that the trademark is held by Software Freedom Conservatory Inc.

I invite you to repeat my search. Unfortunately, the USPTO trademark database search interface is not bery intuitive, so be prepared for some trial and error if you do.