For me, I know to use subversion and git to a degree that I am really comfortable with (from the two I stronly prefer git). Another VCS means that I have to take a few leaps, new commands, subtle to extreme differences in workflow as well as different names for the same things. So a VCS that is well-known by people will (on average) make contributing more convenient to the average potential contributor.
After diving into Emacs' codebase, changing or tweaking a few things that bug me, there are a few walls to climb when actually contributing those changes. I.e. cleaning up the code, creating a patch/pull request, outlining changes and intentions, etc. An unfamiliar VCS adds another burden to the contributor. Remember that we are not talking about people who are paid for diving into their employer's VCS but about people who primarily work on other projects.
After diving into Emacs' codebase, changing or tweaking a few things that bug me, there are a few walls to climb when actually contributing those changes. I.e. cleaning up the code, creating a patch/pull request, outlining changes and intentions, etc. An unfamiliar VCS adds another burden to the contributor. Remember that we are not talking about people who are paid for diving into their employer's VCS but about people who primarily work on other projects.