Many of the same manufacturers supporting CarPlay have already committed to Android support as well (http://www.openautoalliance.net/#press). I would imagine that Apple building on top of QNX allows for the car manufacturers to standardize on a single hardware platform across all mobile integrations.
I doubt that Apple could announce so many automotive partners if the system was completely Apple proprietary, due to both time and flexibility.
I doubt that Apple could announce so many automotive partners if the system was completely Apple proprietary, due to both time and flexibility.