The reason for this is that Apple has managed to argue that its app store is not a monopoly. This is false (If developer accesss to apple and androids app stores were competing even the tiniest bit, then you should mainly see developers releasing apps on one platform and not the other).

in essence, this means that in order for developers to achieve negotiating power parity, they would need to form a cartel, (an illegal anticompetitive practice they would not be able to defend). As a result, Apple's negotiating power advantage is a matter of law.