Also consider the psychology of this.

Human nature means overall we as a group are averse to changing.

Real change follows a certain path.

1. Early adopters

2. General social agreement about the change

3. Wide adoption

4. Full adoption

The problem comes between 2 and 3 - apart from the early adopters, people are very cautious, so this step takes ages. Also it is measured not in time but in iterations of whatever the change is - so if the average person keeps their car three years, and takes two iterations to change, that's a six year delay.

By putting a deadline on the change, it strongly encourages people to make this step as they no longer feel it is such a risk and it increases social pressure.

It also helps solve the problem of holdouts between steps 3 and 4, although in this case the fact that old cars are still available probably means the gradual lack of old-style fuel may be more of a push in the end.