I’m not convinced that it’s the size of the vehicles that is a huge factor because all American auto manufacturers make midsized and compact vehicles. I would imagine that the fact that our cars are left side drive is probably the biggest factor. I don’t think American car manufacturers are making right side drive vehicles anymore. I remember seeing some back in the 70s but not any within the last 20 years. I could be wrong, I’m pretty much speculating based on not having seen and American made models made in the last 20 years that are right side drive.
A lot of it is the size of the cars, our midsized cars are typically considered large by EU standards. Our small cars have tended not to be great -- in fact, the better small US cars have tended to be European or Japanese, such as the Ford Focus (designed in Germany and Britain), the Chevy Aveo (designed in South Korea), etc. More than that, however; our engines (typically a 1.6 or 2.0L standard engine for compact cars) are much larger than similar sized cars in the EU (1.0 or 1.2L standard for compact cars). In much of Europe, the car is taxed based on the engine size, so they don't want the larger engines.
As was noted, emissions standards are different -- with different priorities. For example, in the US we try to eliminate "local pollutants," such as NOx emissions (part of why we don't have diesel engines in cars anymore, but they are still popular in Europe), in the EU they prioritize "global pollutants," such as CO2. Also, there are different safety requirements -- such as standards in the EU to limit damage to pedestrians if they are hit by a car which we don't have.
It's also worth noting that while we don't have a tariff on cars, we do have some high costs related to getting emissions testing -- and so many European cars don't come to the US because of the costs involved in getting the car certified to be sold in the US. While not technically a tariff, these costs prevent many car models from being brought to the US. There are people that claim that rather than having tariffs, we use the emissions and safety regulations, particularly the certification costs, as a way of limiting the number of cars imported.
And there are some American cars made with RHD. While Ford has a plant in Europe that supplies most of their European vehicles, they do make some things like the F-150, the Bronco, and the Mustang Mach E in RHD versions. It appears GM also makes a few, such as the Corvette, the Lyriq, the Silverado and Yukon. Though both GM and Ford have European and Asian divisions, with cars designed and made there, though GM cars in Europe then to be sold as Opel and Vauxhall cars.
