Why? Open sourcing GH would probably be a huge undertaking for a considerable amount of their team (removing proprietary code, making sure no credentials are in there, etc).
Not to mention the fact that the whole of github is probably a bunch of disparate little project worked on by individual teams that would require a lot of effort to turn into something that could be run by an individual person. Probably makes sense to expose some of those projects on their own as open source project like they are doing here.
