Git is fine at scaling horizontally (many repositories), so things like GitHub work well. Neither Git nor Mercurial do well at scaling to a single massive repository; getting Mercurial to do so was easier because we can implement it as out-of-core extensions (see http://bitbucket.org/Google/narrowhg, for example).

There's also internal-only things to deal with, such as the CitC integration. Because of some of the design decisions behind CitC, storing a typical .git or a .hg directory in CitC is essentially impossible. Mercurial's .hg directory is intentionally a black-box - you interact with the repo using the hg tool. The .git directory can be seen as an API - there are at least three implementations that matter for our purposes, and if we change something in the .git directory, we lose the editor/IDE integration powered by the ones we didn't fix (or we did fix, but they haven't been released yet; some of those are linked into products that have a commercial release cycle).