Not everyone can afford self driving cars, and immediately making a thing as widespread a thing as driving cars illegal would have disastrous consequences.

This would result in hundreds of thousands of illegal machines on the road and would a legal and infrastructure nightmare. Most people also would not be able to simply go buy a new car, and even more would still not have paid off their now illegal car.

This idea would take decades to roll out

There are many unanswerable moral debates as to what a self-driving car should do in certain situations and an individual may disagree with the morals programmed into the car.