I am surprised no one has mentioned Github as one of the biggest reason for Git to gain popularity. They pushed git mainstream.

Github launched in April 2008 and within 1-2yrs, they gained popularity. And then when you see a sudden explosion of git/git-core usage is primarily due to the 2Million github users and their 3.7million repositories. Github made git easy to use. Bitbucket was there, but github made it effortless. I am sure if github guys picked Hg in place of git, we should have seen a same surge in usage of Hg.

Analogy can be: Canonical:Linux::Github:Git