This isn’t only a US problem though. They cheated in Europe as well, apparently.
The diesel engines of today are far cleaner and more efficient than any other internal combustion engine on the world market.
This has nothing to do emissions or enviromental quality.
Taxes on fuel (federal, state, sales, etc.) are the prime motivator for keeping fuel economy down, and fuel demand up. Fuel economy is the bain of the feds and oil industry’s existence.