Imagine if you're in a family mini van and you have a Cali governed and you're on a Nevada freeway going "max speed" and the speed limit suddenly goes down from 75 to 55 and your governor puts the brakes on for you to slow you down and you get hit from the rear by a semi from Arizona (without a governor) who can't slow down as fast as the governed "crippled" mini van....because you could not get out of the way driving a vehicle unsuitable on non-Cali roads, your mini van was struck, left the road, crashed in a ditch, 2 parents and 4 kids killed.
If you put the governor on that mini van or you manufactured that governor, you can expect at least 10 years of $$$ multi-million civil litigation and lawsuits and the state of California won't help you out one bit.
Ultimately, nobody will buy a governed vehicle and the prices will plunge, state will bans governed vehicles from certain roadways, people will tamper with their governors, and the entire costly stupid disaster of an idea would be abandoned yet again (for the 3rd time).
Unlike the FAA which mandates RID to be used on all drones going forward (no grandfathering), it is impossible for Cali to require all cars on the road to be governed or else they are prohibited which is just one of a dozen things you need just to make this viable.