One major advantage seems to be the lightweight branching. Take this inflow work I am on with, to share this work with say David I can either check in to the 4.x branch or create a new branch to do the work till we are happy then merge my changes back in. Probably a similar process for both but swapping between branches is heavier on svn (from what I understand).
Take the scenario David is suggesting too. In this case he could have submitted a pull request to do the inflow for us ;) and we could have worked through that way iteratively. I think it is possible to do some/all of this with svn but it seems much heavier.
Plus github is a fast server, my experience of svn.jboss.org has been it is slow, but maybe I just check out behemoth projects.