If you can say "easily" for this, you can't have tried actually importing the OpenBSD CVS repository into git. cvs2gitdump doesn't do too bad of a job, but doesn't attempt tags and branches. I haven't found another conversion tool that gets anywhere close.
I'm not suggesting that CVS to Git is easy. I'm just saying that Github doesn't present any kind of barrier to adoption of Git (in any project, not just OpenBSD).