First of, this is Gran Turismo we're talking about. I don't get how this is in any way, shape or form related to the support a young driving talent gets in Europe compared to the US. Because I kinda doubt that that has any impact on Gran Turismo time trials. For the overall qualitiy of race drivers, maybe. But most certainly not for the times posted on GT5's leaderboards.
Same with the cars and road structure in a given country. I doubt that it matters whether you're used to an auto daily driver or a manual one, simply because the difference in shifting is quite huge between raciing on a track and driving in traffic to visit your grandma.
That said, this reminds me a lot of a question that arose in the StarCraft community: Why are Korean people so good at StarCraft? Simply put, because a lot of them put a lot of effort and time into it. And I for one am fairly certain that the same is true for Gran Turismo and this 'America vs. the Worl' thing.
In the case of Korean StarCraft players, it might very well be because of the way people look at StarCraft over there. If you're good at it, it'll get you money and chicks. Quite an incentive to practice, if you're asking me.
On the other hand, if you started boasting about your mad SC skillz over here in Germany, people would laugh at you, for the overwhelmingly most part.
And that's due to the fact that the game isn't popular over here. In Korea, StarCraft gets a lot of attention from the media, much like soccer on Europe.
And I for one am willing to bet that it's actually the same (to a certain extend) with Gran Turismo. If road racing isn't very popular in a given country (and judging by some previous posts, it's not very popular in the US), people won't start to flock to a game that's focusing on it. In my opinion, that's about all there is to it.