There are several reasons. In the beginning, hg was critisized to be slow as it was written completely in python. The effect of the Linux team going with Git instead created a lot of attention to git, probably also a lot in people who were faszinated by the capabilities of git and did not care much about user friendlyness. The biggest push for git undoubtedly came from github. That is now the premier platform for hosting software, especially open source software.

GitHub is the reason. Turns out giving beer to developers globally is an effective way to get a technology adopted.

That is solely because of network effects. Anyone of them could be the popular one, as could any of the weirder ones like darcs, even it having technical flaws (mostly fixed by now).

The Torvalds effect.

Git and Linux go hand-in-hand, so there is your killer use case right there.

This also shows that quality of the technology matters. Like the discussion on the business value of Amazon's "use APIs always," using git is using a superior source code technology, designed from the ground up for distributed development by a master of distributed development; good things are enabled automatically. Linus' naming and UX choices and inconsistencies aside, the tool is awesome. That's why it wins in distributed development environments - the bazaar not the cathedral and not the bespoke engineering team hidden away in the corporation.

Although it sounds like a silly reason, compare the names themselves. I don’t even know how to pronounce “mercurial” without looking it up. The word doesn’t exactly roll off the tongue like git does. And then the command itself is “hg”. WTF is up with that? I mean, ha-ha we all get the joke, but was the program made for chemists? Unnecessarily clever. Don’t underestimate the extent to which a difficult/confusing name/brand can harm adoption.