Well, I don't think it's mere "narrative". I'm sure Git has its merits--performance, for example--but I seriously doubt this factored into most orgs' calculus considering most orgs probably wouldn't have hit any serious performance issues with hg. I strongly suspect that Git's popularity has a lot more to do with GitHub (and to a lesser extent, the popularity of the Linux kernel) than it did to any actual merit, but I don't think there's any evidence one way or another.
Mercurial had its own GitHub, though. Was it bitbucket? I don’t remember, but it had an equivalent, I think. I really think Linux and performance was a key factor.
The version control software market has evolved a lot since Bitbucket began in 2008. When we launched, centralized version control was the norm and we only supported Mercurial repos.
Github was launched in April 2008 [1]. Bitbucket is harder to find, but was definitely around by June 2008 [2]. Mercurial vs git was definitely the defining difference between the two initially, though the Rails ecosystem exploding onto Github made a huge adoption difference. git being faster + built in support for rebasing was big was well.
