"It is unclear if Apple needs more time to finish developing next-generation CarPlay, or if car makers have decided not to adopt the system, or something else."
CarPlay 2.0 required complete operation in Apple's design as well as full access to all vehicle data.
Put simply, the manufacturer delivers the car but for the driver it feels like he is sitting in an Apple car.
Perfect for Apple, of course. They only provide the interface, the carmaker does the work. And it is not out of the question that Apple is also demanding free access to evaluate the vehicle data.
In short, what advantage do carmakers have by cooperating with Apple?