I believe it's not directly linked to iMessage.
Majority of people believe that iPhone is simple, secure, has the best camera and respects your privacy while Android is the opposite. Also, when did you see a bad guy use an iPhone in any movie? You didn't, because Apple has a strict policy about it's public image; they are the good guys.... the only good guys. Buy iPhone an you will be good, safe and popular.
They are the best at marketing. But also, iPhones are a lot more reliable than majority of Android phones and they have a global logistics/repair network.. I guess the defragmentation of Android market plays a big role here.