Basically, it looks like the developers don't see any benefit to migrating that outweighs the cost of a migration. [1]
There is a GitHub mirror [2] maintained by, I think, one of the developers, but they don't accept pull requests.
It's interesting to note that, for such a popular project like tmux, there are so few contributors. [3] Five, to be exact.
Maybe that's an artifact of how the mirroring to GitHub works (perhaps the commit authors aren't translated correctly?), but I suspect it's more an artifact of an outdated contribution process and tooling.
[0] http://sourceforge.net/p/tmux/mailman/search/?q=github
[1] http://sourceforge.net/p/tmux/mailman/message/33826395/
So I guess something just didn't translate correctly to Thomas's GitHub mirror.