The issue is that you run into a point of diminishing returns. Compared to cars of the 50's and 60's, today's cars are something on the order of 95% cleaner (maybe even more) That's pretty good. Is it worth it to pay another $3k-$5k per car just for another 2 or 3 % improvement? I don't think it is. Emissions from industry and even agriculture are more prominent than emissions from cars/
If you're computer savvy it's kind of like the amount of RAM you have. If your computer has something like 2GB or RAM, then going 4GB will yield a massive performance improvement for $20.. If you have 4GB and you go to 8GB of RAM, you'll see a noticeable improvement, this time it cost you $40. If you have 8GB of RAM and you go to 16GB, most of the time, there's not going to be a notable difference, and you're paying $80 for it. If you have 16GB of RAM, and you upgrade to 32GB, there's going to be no discernible different at all, and you're paying $160 for that.
The point is, at some point you get as far as you practically can (for the time being) with one source of pollution, It's more productive to look towards other sources that perhaps haven't been under as much scrutiny as the automobile, even if it's not politically correct to do so. How much pollution do you suppose a couple hundred thousand two stroke mopeds make in comparison to an equal number of modern Priuses?