I remember the GitHub team investing huge amounts of effort into convincing people to use git and teaching them how to do it - one of the four co-founders (Scott) was dedicated to that effort full-time.
I started using git when I was working on embedded Linux professionally in 2005, witnessing the whole bitkeeper saga. I am keenly aware of the history of DVCS 15 years ago.
Meanwhile outside the bubble of academia and startup web devs in 2008, Windows was still by far the most widely used dev platform. I typically deployed Mercurial when I wanted to convince a wider technical audience others of the beauty of DVCS.
GitHub felt safe the same way Instagram (among many) felt safe deploying for iOS only, even when then Android had a larger user base. There are other factors at play, and it was not that git had some kind of major advantage in 2008. If anything mercurial had a slight edge. But if anything in 2008, the wider DVCS market was still in its infancy.