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.
The vast majority of offroad equipment is not farm equipment but operates in urban environments. As NOx is an air pollution concern, there should be different regimes for rural areas versus urban areas. Construction equipment operating in urban areas is different from a tractor on a farm.
