GitHub is unable to enter the mobile app distribution market because Apple and Google leverage their monopolies in the mobile OS market to limit competition, or outright ban it, in the mobile app distribution space.

It'll take some serious antitrust action before real competition will be allowed to improve costs and experiences for developers and users alike when it comes to mobile app distribution. In the meantime, Apple and Google will continue to keep a stranglehold on the app distribution market like they have for over a decade now.