If everyone wanted to save the world right now then they would just get the clean diesels. But then that makes diesel prices go up, then they have to switch over to produce more diesel making gas go even higher. And it will take a lone time for everyone in America to switch over.
Every solution has more problems that come with it.... Well i shouldn't say every solution.
market will pick what is best if left alone... governments will not. free market will allocate scarce resources most efficiently