The problem with "only one per country" is that the US is so much bigger than any country in europe. With one race, only a small section of the country can reasonably consider actually going to the track.
Granted, other large countries like China and Russia only have one race, but do they also have the market potential the US does? Obviously China doesn't. Probably not Russia either.
I get what your saying but I think on the whole ,americans tend to make their own sports up and stick to them.They're not like the rest of the world where most play football,F1,cricket,rugby and so on.You have the likes of the world series baseball that only includes american teams.NFL that they class as world champions,but again only they play it.Basketball are very limited to a few counties around the world.
Indy car,nascar again made up themselves.I just don't think it's in their culture as a whole to take something new up made up by outsiders.A lot of americans don't have passports or even leave the US in their lifetimes.When you have a country that size it's understandable,great country after all.
Please don't take this post as a dig at americans,it's purely ment as I see it from a sporting prospective.Clearly there are a few members on this forum for a start that brake the rule,but not many do from personal experience.