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.