I see a lot of answers to this that rely on the feelings the author had when hearing about one or the other SCM. Others say it all was sheer luck. I believe luck can be traced back in history.
I will talk about history.
Git and Mercurial were created simultaneously in order to solve the same issue. Back in those days, the Linux kernel was forced out of a period during which it had been using BitKeeper, a proprietary distributed SCM which had served them well for many years.
Indeed, Larry McVoy, CEO of BitMover, the company behind BitKeeper, stopped giving his software away for free to Linux developers, because someone inside the Linux community had reverse-engineered it.
Linus Torvalds, dissatisfied with what already existed, subsequently started to work on a brand-new SCM that he would soon call Git. Quickly thereafter, Matt Mackall started the Mercurial project for similar reasons.
After some time developing these projects separately, Matt Mackall presented an advanced version of his SCM and benchmarked it a certain way, comparing it to Git (which was itself only a couple weeks old). Linus considered using it instead of Git for Kernel development, but dropped the idea when he realized that Mercurial was using Changesets to log revision modifications. He feared that was too close to the way BitKeeper worked, and he certainly didn't want anything that could make someone say, "They built a BitKeeper clone".
Git was therefore used for Kernel development instead of Mercurial, but both were technically relevant. The end-result is, Git started out by being actually used where it was designed to be used, while Mercurial was not as fast to find its first big FOSS use. Because it was endowed with a very good design, and thanks to Matt Mackall's perseverance, it eventually became famous and got used for big, real-world projects.
Today, they are both famous. Which one is most famous is impossible to say. Google Code only integrated Git recently, while it had Mercurial for a long time. Many really big and famous projects use either.
I guess what I mean is, when the very reason why you have started a project vanishes, it is harder to gain popularity, but still feasible.
Bazaar is another SCM which is very famous in the GNU world, but not so much outside that, because it was built with the intent of satisfying the GNU community. Software often go where their creators want to go, and no further.
On the other hand, distributed SCMs are clear winners. I don't see many widely-used non-distributed SCMs out there.