John, I totally understand where you're coming from. But here's the thing: MASON is largely coded cathedral-style. The overwhelming majority of the code is presently written by me or by a small coterie of coders here at GMU. This may not always be the case but it has been for the last many years. And distributed VCSes are useful for bazaar projects, but they pose very few advantages, and indeed a number of disadvantages, for cathedral projects.
But in honest the big reason is: I just can't be bothered to pick up Mercurial at this time, much less git. It's just too many neurons which I could be spending on other causes. SVN has served our purposes pretty well and we've resisted the occasional calls to jump to something else (and at any rate if we did jump, it'd probably be to mercurial, given that hg is better supported on google code).
That being said, if the community changed and we had a lot more contributors doing contrib packages which required tight integration with the system, then branching would become a bigger deal. But it's not happened yet.