You know, I actually expected that the opposite would happen: GitHub would start offering Mercurial hosting. Because Git isn't what attracts most people to GitHub. It's the sheer fact that GitHub is frickin' HUGE and has lots of people who will show up, fork your project, and send you a pull request. I love Bitbucket, but honestly if GitHub added Mercurial support I would probably move all the way to GitHub because o…
They're insanely popular and becoming more so by the minute, and their biggest problems probably have to do with managing growth, not lack of users. With git itself becoming increasingly popular, there's absolutely no sense that git will be a limiting factor any time in the future.
They clearly do recognize the long-term advantages in improving the usability of git, and have been making moves in that direction (iphone client, increasing the functionality of their web interface, some interaction on the git dev list), but hg's purported UI advantages are tenuous at best, and it seems a much surer bet to consolidate their position and improve git instead of investing a lot of effort in supporting hg, which would only _dilute_ their position.