Goodbye Bitbucket!

vot

Of course Git is quite good and very popular.
But in short:
1) Mercurial is simpler in learning than Git
2) Mercurial is better in branching than Git
3) Mercurial is more scallable than Git
4) Mercurial have the best GUI among all others

And yet another argument in favor of Mercurial:
FaceBook development team totally migrated from Git to Mercurial.
Some time ago, Git was recognized by FB engineers as unsuitable for scaling their projects.
FB guys share their experiences with using and scaling Mercurial.
They say that now their HG is sometimes 5, or even 50 times faster than Git.

Scaling Mercurial at Facebook

By the way, Python language development was migrated to Mercurial too yet in 2009.