EU emissions standards are usually more tolerant than USA standards. The problem with American cars sold to Europe is that the EU has stricter pedestrian protection requirements which would require redesigns of American models. It makes sense not to implement those when designing a car for a country where crossing a road is illegal in many places, but inside most of the EU that's unacceptable.
Other safety standards have also been lower in the USA but I'm not sure if that still holds up today.
There's also the space problem: many American cars have trouble fitting onto European roads and parking spaces because the USA has dedicated a lot more space for cars as a result of city design often practically forcing you to own a car. The EU has its SUVs but most of them are small in comparison to the unimportable American ones,
Other safety standards have also been lower in the USA but I'm not sure if that still holds up today.
There's also the space problem: many American cars have trouble fitting onto European roads and parking spaces because the USA has dedicated a lot more space for cars as a result of city design often practically forcing you to own a car. The EU has its SUVs but most of them are small in comparison to the unimportable American ones,