And specifically it includes a drupal-org.make file which is a manifest for the Drupal.org packaging system which bundles up this profile, all of it's dependencies, and Drupal core into a single download. Perhaps they are planning to put it there, or just decided to mirror it on Github for now.
Also FWIW, the petition namespace on d.o is already occupied, perhaps that is blocking the release on d.o?
A drupal-org.make file, like any .make file that the Drupal tool Drush uses, is independent of the Drupal.org packaging system.
There's a lot of pressure from within the Drupal community to host all Drupal contrib modules, themes, and distributions on drupal.org but I don't see anything wrong with a distribution being hosted on GitHub. It certainly brings more visibility to the project from developers outside of the "Drupal Island".
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.
Also FWIW, the petition namespace on d.o is already occupied, perhaps that is blocking the release on d.o?