I think it is also a trust issue - GM doesn't trust Apple will approve updates to their iphone app and so won't risk it. Also Apple charges a lot of $$$ to car company to connect to carplay and I think GM called their bluff. I'm not sure what the costs to android auto are or why GM wouldn't still connect to that.
Note that as a consumer I don't trust anyone above. Sure GM's app will work great now, but in 3-6 years I expect they will decide drop support for my currently working car in their app. Apple and Google will also change their OS and so the app that does work with my car won't work anymore on new phones. I also expect updates to CarPlay/Android Auto that won't work with my old car in a few years. Nobody in this business wants to provide support for the average car which is currently 12 years old - unless you pay which someone driving a 12+ year old car won't want to do. (GM provides parts for those older cars, but people pay for those parts)
Note that as a consumer I don't trust anyone above. Sure GM's app will work great now, but in 3-6 years I expect they will decide drop support for my currently working car in their app. Apple and Google will also change their OS and so the app that does work with my car won't work anymore on new phones. I also expect updates to CarPlay/Android Auto that won't work with my old car in a few years. Nobody in this business wants to provide support for the average car which is currently 12 years old - unless you pay which someone driving a 12+ year old car won't want to do. (GM provides parts for those older cars, but people pay for those parts)