In the 50’s and 60’s, Europe was at a large economic and exchange rate disadvantage to the USA, due to the aftermath of the Second World War. The US automotive industry was focused on US markets then too, and not suffering yet from competition with the Japanese or Germans (though American people were buying VW’s).
After the 70’s, American car body styles started to seriously suck (in my opinion, though so did other nations’ car body styles).
I imagine a lot of the “reason” is just circumstances and perception. US companies don’t care (or don’t do a good job of researching and understanding foreign markets and adapting to them, however you want to put it) about foreign markets enough to make themselves really competitive around the world, and Americans have been conditioned to want different things. Americans supposedly tend to want automatic transmissions, huge “comfortable” (as in, you are driving two or more giant sofas around, now with TV’s for everyone and other comfort features), high top speed and brute force rather than balance or efficient torque, there are giant lanes and massive parking lots in the USA, and many modern American homes look like a set of giant garage doors and you may have a hard time finding a front door because people drive in and enter the house through the garage.
By contrast, you have the Prime Minister of France wanting to outlaw SUV’s from the streets of Paris on the grounds that they are non-car abominations. :-D
Also, I’ve driven big American cars (and others) in the US, and I’ve driven in Europe. Imagining trying to drive in Paris, Rome, Milan or Florence, for example, in an American “land yacht”, I think it would be difficult to maintain the awareness and agility needed to flow with busy traffic that is flowing and swirling and using small spaces of opportunity. At least your yacht-car (taking more space than some European trucks) would tend to win collisions, but it doesn’t sound like fun to me, except if you wanted the attention.
Parking would often not be fun, either.