I was a late adopter - switching from Mercurial to Git in around 2010.
The reason I believe Git became so popular is because of sites such as GitHub you had a network effect in version control tools. This was previously not seen, as you would share code on a project or company basis.
I specifically remember switching to Git and Github because all the projects that I was interested in following and contributed to had done the same, as well as developers that I associate with.
That is a network effect.
GitHub was the most popular web based collaboration layer built on DVCS and Git ended up being 'good enough'. Mercurial was certainly easier to learn and use, Git has many nuances, but had a solid brand because of Linus.
Just because GitHub launched in '08 and the growth begins in '10 doesn't mean GitHub isn't responsible. If you look at competitive growth charts in other areas such as social networking and the growth of Facebook the line is very similar.
You don't see growth charts like that without a viral loop / network effect.
For eg. compare to a chart of Facebook growth
Update: I know the source above may not have been accurate, but there are plenty of data sources that demonstrate that Git has been growing exponantially in the past few years.
Graph 1: Mentions of Git in job ads
And the Eclipse survey which shows that Git market share went from 13% in 2011 to 27% in 2012. Amazing growth.
This post makes a much better explanation of Git growth and network effects than what I have done here.