It's a bit odd that there's such a monoculture around git though, where are the competitors?
I might worry about a monoculture around a hosted service like github, but not git itself.
It obviously isn't 'factually' better - that's a matter of opinion not fact.
And in my experience, both Mercurial's model and UI is very over-complicated with far too many concepts. Git has a smaller number of simpler primitives and primitive operations. I think that makes the underlying model of Git better and that's why I prefer it.
Or consider openssl since libressl came out. Yes it's possible that codebase improvements would have happened anyways, but having open source competition is great.
As it stands, it's just a lot easier to use Git everywhere though, just so developers don't have to learn a new tool.
https://pijul.org/ is the only somewhat somewhat interesting alternative I know about, but it's implementation is in alpha territory, and development seems dead.