One reason I don't see mentioned here is the perverse incentives to manufacturers to make larger cars due to CO2 emissions regulations differing depending on the size of the car. So instead of making more efficient engines to hit the targets (no doubt the intent of the regulations), they just stop making small cars.
If they had intended to reduce CO2 emissions consumption, they would have just increased taxes on things that cause CO2 emissions (e.g. a tax based on distance driven in a car that emits CO2). Or even easier, increasing taxes on fossil fuels.
However, the intent was to say they (politicians, society) did something about CO2 emissions without actually giving up anything. Which was accomplished.
One reason I don't see mentioned here is the perverse incentives to manufacturers to make larger cars due to CO2 emissions regulations differing depending on the size of the car. So instead of making more efficient engines to hit the targets (no doubt the intent of the regulations), they just stop making small cars.