The bureaucracy is probably the larger hurdle. Snartphones don't really risk lives the same way cars do. And there are still a lot of people wary of the safety and capability of self driving cars. It could be hard to convince a city to agree to having them, and convincing every city in the US may take a long while.
When they are proven to be considerably safer (a question of "when" and not "if" IMO), I highly doubt many politicians will want to stand in the way of allowing them on their streets... nevermind if Apple ever comes out with a car, it could quickly become a highly sought after experience.
Adoption was also spread unevenly. Some parts of the country moved past flip phones long before others. There are still holdouts!
Cars will be an entirely different matter. People are trained to accept internet enabled devices and ridesharing already.
Once insurance rates for manual navigation clear those of an autonomous fleet vehicle, people won’t want the oil can Henry’s experience anymore.
You can already tell most people don’t want to drive anyway. They are looking at their smartphones as they idle down the street.