The short version is: the PC is a historical accident. By "the PC" I mean "the Windows-Intel platform on which most consumer PCs were built." Linux and BSD were both able to exist in the form they did because there was a commodity hardware platform that was standardized (ad-hoc standardization, mind you) and _somewhat_ open. IBM, Microsoft and Intel were all best frenemies, able to exert enough power to standardize the PC platform but also able to exert enough power against each other to prevent them from locking the platform down too much. There is no standard "smartphone" platform like there is with the PC, really the only standard is Android AOSP. Because of this, it's a lot harder to do a third-party phone platform without adopting large parts of Android's code.