That's technically and conceptually easy but it costs a lot of money. One approach would be to put a large DC-DC converter on the car and run a 48V bus next to the 12V one. That's a very expensive device, needing to be perhaps a few kilowatts depending which devices are on 48V. It's also a lot of extra wiring, and the need to manage the extra variations in vehicle systems.

A second approach would be to put a DC-DC converter only in the devices that need 12V. But that's asking every supplier to change their design to accommodate 48V and the vehicle manufacturers will bitch about the cost increases. It'll be a moderate increase across a LOT of devices.

I'm working on a product that uses a highly integrated SoC that has all the regulators needed to connect it directly to the 12V line (it'll run on 6-20V or so). It even has an on-board 5V regulator for additional off-chip devices. The best thing for us would be if the same chip existed but could be connected directly to 48V (not sure what the range is on that) but from a silicon standpoint there is a big difference as well. Adding an external regulator would add cost, and invalidate some of the wake-up features and low current standby capabilities of the chip.

There are always "reasons". The only way to make the transition is for someone to make it happen, but again the suppliers who participate may be seeking premium pricing in the beginning because there aren't options.