Last time I checked every alternative to Git was lacking some major feature that would immediately make it a non-alternative to me. (And I wouldn't even remotely call myself a Git power user.)

For Mercurial, that was shallow clones. I haven't looked at it in a while, but a quick googling shows me a wiki page where they discuss how an implementation could look like, and it hasn't been updated since 2015.

Network effects are certainly a reason git is popular. But also: If you want to challenge git, you should be able to compete with its basic features.