By the same token, why have multiple programming languages? We can just use C. Why have different operating systems, we can use Windows for everything.
I'm actually super happy about mercurial support. Git is very complex and that complexity is a real barrier to entry and a time suck when a beginner invariably screws up. And if you feel comfortable with your git Fu, please explain to a beginner how to recover lost commits using git reflog or when you would use an octopus merge.