In fact the Iphone and Itouch are the dominant devices where people buy games, music, videos by using itunes.
Others devices don't have such software and download/buy platform !
That the things that make Iphone so succesfull.
Unity team have understood that and have incorporated it.


Games for other phones have market, but dont' have the same selling power as itunes platform !
And on other devices, lot of times you will have to do 2D games or very simple 3D games.


Another way to develop is to use Java language, caus lot of them are Java compatible.
And for that you can find some good 3D engines already existing like for example Jmonkey Engine : jmonkeyengine