The main reason OEMs chose Android is because it is released under a
non-copyleft license (which means they can change the code without releasing
their changes allowing product differentiation).
The secondary reason is because Google managed to attract developers to their
proprietary app store (Market) which made Android along with the Market an
attractive platform for consumers since they believe that whatever common
tasks they want to get done with the phone they will be able to find an
application (among the tens of thousands) which does it. Of course the number
of applications is a pointless statistic since most of those are useless
(e.g. fart apps) or simply shortcuts to websites.
Also, GNU/Linux for phones is indeed technically superior to Android for
non-phone related tasks (e.g. it has real multitasking and package
management). There are also several phones that come shipped with GNU/Linux
such as the Neo Freerunner (which uses only free software drivers) and the
GTA04 (which is mostly free software compatible also aside from proprietary
firmware required for wireless and bluetooth).
