Except that the refresh cycle for a mobile device is around two years, and word spreads more quickly than ever that you can't run the latest stuff. If that Android phone can't run the latest apps, lags, or can't do all the things that your friend's iPhone [1] can do, easily, and reliably, you're a lost customer. On the flip side, if you're a developer with limited resources and your app can't run on a vast majority of deployed devices without extensive and expensive testing, you're a lost developer. Neither of these things is good for a computing platform. Never was, never will be. These are first principles. Sure, there might be other factors, (which you haven't specified by the way), but all else equal users and developers want consistency in a platform for the sake of compatibility.

1) Top rated review for Path for Android: http://cl.ly/1P082D1L2D1k1O0i3K0J