Support is very expensive. Say what you want about Apple, but they provide absolutely stellar support, especially with the stupidly inexpensive Apple Care insurance. This is only cost effective if they can make reasonable predictions about how their devices will behave in any given scenario. Interfacing Apple hardware with non-certified (MFi, BLE, etc) third party hardware has a non-trivial risk of unpredictability high support costs, either from excessive Apple Care claims, customer support communications, or just overloading the Genius Bar.

Reducing support cost could easily explain the motivation of the entire walled garden if they are sufficiently high.