Because everyone else is still using git.

There is a Mercurial extension that allows you to use hg commands to interact with a git repo

https://bitbucket.org/durin42/hg-git

My experience with hg-git has been that it is not always a great citizen with git repos and it can cause something of a mess. (This is a few years old now, because like the rest of the world I switched to git; it may have improved in the meantime.)