At first, reducing car pollution is fairly straightforward, and there is a lot of reduction for the cost. But the cost of reducing emissions grows as the efficiency of current pollution control measures increases. There eventually comes a point when the cost of further reductions in pollutants becomes prohibitive. I suspect that with car emissions, we have reached that point, so that it costs more and more money to achieve ever smaller decreases in pollutants.
One would think that the magnitude of the cost would outweigh any fractional gain at this point—but then, these are politicians making the decisions.
Letting a bunch of people who don't know how to do anything themselves make all the decisions and boss everyone around, without any accountability -- that's the important thing. ;^)