Don’t even have to go that far back. Mercurial is/was a way better ux over a very similar feature set.
It's especially vexing that mercurial throws a fit if you try to do anything like pull with changes in your repository while git does the sensible thing and just works.
I think it was the performance.
It was GitHub. GitHub changed it from "use one of several analogous products"[0] to network effects.
[0] Joel, founder of StackOverflow, had a GitHub like competitor that used either git or mercurial commands on the same repository. Just as one example of how similar they are.
Regardless of what the benchmarked real-world performance of Mercurial was or how well optimized it was, there is a class of developer that thinks all Python is slow and may never change their mind about it.
Python seemed a good decision: git had a two week headstart and Mercurial was faster to hit many usability milestones (and arguably git may never hit some of them, jk). Mercurial had good Windows support from early on (and doesn't need to ship like half of the GNU userland to do it).
I just don't think it should be surprising that some Linux kernel developers dismissed Mercurial off-hand just for being written in Python. I also don't think it is a coincidence that early GitHub ignored Mercurial just about as dismissively. (Ruby and Python aren't entirely "competitors" but there isn't always a lot of shared love between them.)
Whether it was the language or not, Mercurial was objectively slower than Git.
Nowadays that doesn't matter much, but 15 years ago, it was a much different story.
> Whether it was the language or not, Mercurial was objectively slower than Git.
That was not my experience, but I was on Windows at the time. Mercurial "objectively" had good Windows performance. (Not just because it ran at all, versus how much work it took to tame Frankenstein's Monster of shell scripts that was early git to run at all on Windows. But also because it's file system transaction model always fit Windows better.)
(ETA: Also, to be fair, my "team" at the time was darcs and my opinions on VCS performance at the time were from a very different perspective.)
I see a lot of people attributing Git's "win" to GitHub, and that's likely the biggest nail in the coffin, but the GH devs chose it because it was already reasonably popular and it was popular because of Linus Torvalds.
Because it's called Github, not Mercurialhub ;) (I bet if Github would have chosen Mercurial instead, the popularity would be reversed, at the time I switched to Github I was just looking for an alternative to SourceForge for hosting my open source stuff, but didn't care much about the actual version control system).
IIRC git grep search took seconds compared to more than a minute with hg grep.
One thing that perhaps let me understand git better later was how `hg pull` works actually like `git fetch` and makes more sense than `git pull`.
The team has eventually switched to git anyway.