The answer for why Git beat Mercurial in overall usage more or less boils down to GitHub being very successful. That isn't to say that Git wasn't already in a good position prior to that, but GitHub made it very easy to get into Git in a way that hadn't really existed before. (Anyone who used CVS or SVN on SourceForge knows what I mean.)
At that point, it's just network effect. GitHub made using Git easy enough that people were willing to accept a clunky interface. It's not like people expected great interfaces from CLI programming tools in 2008, anyway. People are still using Mercurial, in any case.
As for Darcs, I used it. It's cool. I'm not convinced it's that much easier to understand than Git, and it has well-known performance issues.