Because the Google Play Store have the most apps and all-time the newest updates and APIs but for amazon etc. android isn´t a free system they haven´t a chance and it is so hard for they to come on android phones. Theoretically, it is open but not in practice only Google is the winner on android phones. I personally love android, but it is true that it isn´t fair that amazon app store, Aptoi etc. haven´t a real chance, I personally like they because they are also great app store.
But be we real Google is the real owner of Android and they will be forever the biggest company on android, only because that, that they made the system. No other company can use android so perfectly like google.
But for the end I have to say android is theoretically open but, in practice only Google is the winner on android phones.