alt Hacker NewsMercurial was the only Python application that worked, so van Rossum and crew did not like it and had to move to git and then GitHub (Ruby).
Python developers, especially core are used to perpetually broken software and don't like stuff that just works. As long as the software is in Python - C (git) and Ruby (websites that used to work at that time) are fine.
Honestly hg shot themselves in the foot by not releasing any stable API and making developers use their CLI interface. Between that and the performance and dynamic typing issues of Python, it was almost sure to lose the race to Git.