[simscitizen]

Because it's fundamentally different and more secure.

Apple's model uses a public/private key pair: the private key never leaves your server and Apple doesn't know it. Apple only knows the public key, in the form of a cert. Apple actually writes about the trust model in the docs: https://developer.apple.com/library/ios/documentation/Networ....

Google's model uses a shared secret (the API key) that both the client and server know.

Having worked with both systems, I prefer the ease of the shared secret model, but each system uses a fundamentally different security model.