Having used ClearCase, CVS, Subversion, TFS, Git and Mercurial, I would have to say it's because not enough people have tried Mercurial. I have no interest in Git, but I think people like GitHub (or maybe don't know about BitBucket)
To be fair, BitBucket was way uglier than GitHub until very recently (latest redesign). That's a turn-off for most people (unfortunately). The new BitBucket is awesome, though.
At least that's how I see it.