http://www.python.org/dev/peps/pep-0374/#why-mercurial-over-...
The reasoning was a bit odd (right at the end). For example:
> First, git's Windows support is the weakest out of the three DVCSs being considered...
True... kinda. In my experience it sucks unless you use Cygwin. I'm not sure how much that matters however. There is excellent tool support for Git on Windows, namely Jetbrains' IntelliJ (with the Python module) or PyCharm. Version control integration is one of those things that Jetbrains excels at with nobody else really coming close.
But, then again, the vim/Emacs Python crowd (over IDEs) are still pretty strong.
I'd be interested to know what the "core developers" disliked about Git too.