That's not entirely true, in the sense that popular editors tend to have more extensions, syntax highlighting for more languages, auto-completion, etc... That helps draw in more users.

I just don't see where people would be having the same experience with Mercurial.

Well, there's bitbucket. IIRC some programming language communities have standardized on even more off-mainstream VCS.

But yeah, on the whole Git got an advantage in users somewhere along the way, and that's now self-reinforcing. It helps that there's no competition that's obviously better.