I just feel like Android is a smarter OS, that's why it's winning out. Compare the changes Google have made to Android in just 5 years, then compare that to what Apple have changed in iOS in 6 years...apart from stealing the Android notification system and adding wallpaper support, they have changed nothing at all. The OS looks dated and old fashioned.
Android is thriving because it's so much more adaptable, it can suit the need of a massive range of devices. iOS not so much.
Apple do make decent hardware, but iOS is so lacking and behind, it says something when Microsoft have managed to become on par with it in just 3 years...Apple have not innovated for a while now. Apple Maps and Siri have been epic flops and lacklustre. Google actually designed something people would want to use with Google Now. Nobody needs Siri. Google just have a better vision and Android is just superior. The figures speak for themselves. If Android was rubbish, OEMs would be running to Windows Phone or creating their own alternatives, but they aren't.
Google keep on hiring the best to work on Android, the UI has improved ten-fold and it looks like they are about to change mobile multiplayer gaming very soon. There is just more potential in Android than iOS, and personally I think that has always been the case. The only issue with iOS is that Apple owns it.