Personally, I chose Mercurial to start with, because I liked the Windows tooling available and it felt a lot more like Subversion, which is what I used previously.
However, Git won the mindshare war in the end, so I moved over to that.
Ahhhh, thanks for brining back those memories.
You're not thinking about BitBucket are you? I thought Github was always git only?
What I mean was that it wasn't like Bitbucket where you chose whether you wanted git or hg for starting your project.
I think Google Code provide both from the early day (along with Subversion), and maybe also SourceForge.
That was over a decade ago, however; I'm actually pretty surprised it took Mozilla so long to switch.
I'm a happy git user but I'm really curious about losing good ideas from other tools due to mindshare.
However the killer feature of Git is the ecosystem. I was always having to do lots of custom work to get Mercurial to work with CI providers, whereas Git just worked and had first class support. It was clear after a while that our team would always be outsiders if we continued down that path, and there wasn't enough of a compelling reason to stay with Mercurial.