I think PR and network effects of GitHub definitely played a role in the success of Git over other options like bzr, but you should also remember that bzr had tons of issues. It was slower, there was no index/staging area, there was no rebasing, etc. Mercurial was very good too, but while there were pluses and minuses with all of them, I think there was a _lot_ of careful comparison too. None of them were clearly and in all aspects better than the others.