It sure had momentum. As did a bunch of other distributed VCSes. If you were a party to voting what VCS to switch to for some of those high-profile projects, I'd very much like to hear about it.
In GNOME, a decision was delayed and bzr and git were pretty evenly matched.
Linux has previously used BitKeeper, but that didn't make it "win out", just like it didn't do so for Git. Sure, it wouldn't have existed if there wasn't a need for Linux.
I am only pointing out that it was GitHub that helped popularize arguably the worst UX among DVCSes: I don't hear people say "your free software contributions portfolio" — they'll just say "your GitHub profile".