I honestly don't know, but is there a reason (other than wanting people to buy a newer model) that "smart" car radios couldn't add this technology retroactively? For example, we have a 2013 Hyundai Sonata Limited that connects to my iPhone, but in a very limited way.
(Note: editing this to include the fact that both bluetooth and USB connections are available in our car, as well as many others from other manufacturers.)
there are multiple things. carplay requires the display to be at least 6.5" diagonally with a specific aspect ratio. obviously you can't change that from software.
carplay runs over IP, and IP connectivity is established with usb tethering in the wired version. many older HUs don't support such functionality, but just USB mass storage, which is perfect for playing media files stored on the connected device (pen drive, usb hdd, smartphone) flash, but not enough for carplay. with regards to bluetooth, it doesn't play no role in wired carplay at all. wireless carplay runs over wifi, but connection negotiation starts on bluetooth LE.
stuff is well explained here: https://developer.apple.com/videos/play/wwdc2016/723/