Amazon probably has a more direct and controlled app marketting system. Like, they'll decide which apps are featured the most - so while Google Play might be a more even field, getting into Amazon's good graces might be the way to hit really big numbers... I mean a lot of people like Kindle, they are very popular and considered more than just an Android tablet.
iOS might be the same issue - very difficult if not impossible for an indie to get to the front of the app store, it's all for EA and their ilk these days. Really, I would suggest that when you make enough money with an App, get a mac and make an iOS version, but only when the costs are covered by the Android version.
It boils down to marketting these days, which sucks because who really has money for this stuff - it kinda detracts from the joys of being an indie developer... I can't see the situation getting any better anytime soon. My iOS game has a publisher behind it, marketting... and that didn't make much difference.
I am the one who knocks...