There are 2 problems comparing diesels in the US and diesels in Europe.

1. Diesels in the US are essentially unregulated. As long as they get the mileage on which their operators estimate their operating costs, they don't care about the soot. There's no legally justifiable reason to pull over a soot-spewing diesel truck or car in the US.

2. This is broader than just diesels, but the geographic size difference between the US and Europe results in significantly different market conditions (hours of driving between major cities, population density outside of certain big cities like Boston, NYC, DC, Chicago, SF).