I would have liked them to move to bitbucket.org because it was built with Django.
But Mercurial itself wasn't (although it was written mostly in Python). I think this is a good example of taking the pragmatic choice (GitHub) over the ideological choice (Bitbucket).
> Why Git/GitHub, as opposed to Mercurial/Bitbucket or some other system? Because it's very well-made, and it's where the people are.
Where the people are would imply because of the social aspects.
Both offer similar feature sets. If I want to keep my fork on Bitbucket, it's easy.