It does, which might have been part of the reason they did it this way. Github is increasingly "the place to be" for open source projects, so the White House probably wants to get involved there too. (I don't mean this cynically BTW--I believe they honestly are trying to move the government more toward open source.)
And a lot of Drupal developers do use Github for their own work. On the other hand when they go to check out distribution options for a new project, I bet they start on Drupal.org. It's just so convenient, and like you say, there's a strong norm within the community to put everything there. Even the few "commercial" distributions can be found there. I assume this work will show up there at some point as well.
