That's just not true. SVN was more popular than git until several years after GitHub was founded. Mercurial was reasonably popular as well.
SVN may have been more widely used, but it was on the way out by the time GitHub was launched. It was the incumbent, at least in communities at the forefront of change. GitHub and integration with Microsoft tools certainly accelerated it's adoption into the enterprise environment, but among the "new VCSs with a future," git had already won.
? Github launched in 2008, git was popular then but not the completely dominant thing it is now.