For me it won (10+ years ago) because for some reason git (a deeply linux oriented software) had better Windows support than Mercurial (that boasted about Windows support). You could even add files with names in various writing systems to git. I am not sure that Mercurial can do that even now.
Mercurial on windows was "download tortoisehg, use it", whereas git didn't have a good GUI and was full of footguns about line endings and case-insensitivity of branch names and the like.
Nowadays I use sublime merge on Windows and Linux alike and it's fine. Which solves the GUI issue, though the line ending issue is the same as it's always been (it's fine if you remember to just set it to "don't change line endings" globally but you have to remember to do that), and I'm not sure about case insensitivity of branch names.
Pretty sure Mercurial handles arbitrary filenames as UTF-8 encoded bytestrings, whether there was a problem with this in the past I can't recall, but would be very surprised if there was now.
Edit: does seem there at least used to be issues around this:
When evaluating successors to CVS back in 2007, Mozilla chose Mercurial because it had better Windows support than git. 18 years later, Mozilla is now migrating from Mercurial to git.
I migrated from Hg to Git a few years back, only because BitBucket forced my hand and most of the hosted CI tools stared dropping support for Mercurial.
But I still prefer Hg over Git.
