Emissions regimes are complicated, but US tractors fall into the much less restrictive off-road category. As a result, they're a disproportionately significant contributor to things like NOx. A long time ago the off-road category was >20%, and I'm sure that percentage has only grown as regulations have forced emissions reductions in onroad vehicles.

> but US tractors fall into the much less restrictive off-road category.

Sometimes. Above 26HP tractors do have to have emissions controls like diesel particulate filters now. Below that they don't.