Well, IMHO, distributed VCS's like Hg and Git are inherently better than a centralised VCS - so SVN was always going to lose out to one of them.
And git, as has already been observed, had the huge advantage over Hg that it was used by the largest and most successful open-source project on the planet - that's a hell of a track record, right from the start.
As to why the sudden explosion in early 2010, my guess is fairly prosaic. Git is brilliant, but it's not massively intuitive for a beginner.
The best Git book, IMHO, is Pro Git, which was published in Sep 2009. The second-best (IMHO again), O'Reilly's Git book, was published in Jun 2009.
So the reason Git usage exploded in early 2010 could be as simple as the fact that that was when really good resources for learning how to use it became available.
