I think there is a big enough population, and enough talent in the US to produce some F1 caliber race drivers. The problem as I see it simply that most American driving talent is funneled into NASCAR, and it makes sense when you analyze it.
1) NASCAR is much more popular in the US, I'd bet an American F1 champion would get much less attention here the the NASCAR champion.
2) NASCAR is easier on drivers/drivers families since it's NA only so they don't have to travel all over the world, and be gone for several weeks at a time like F1.
3) It's easier to get into NASCAR, compared to F1 it has, more drivers, and less people wanting to be drivers (again because it's a US only thing, maybe 50 million US kids dream of becoming a NASCAR driver, while 100's of millions of kids worldwide dream about becoming an F1 driver)
All in all I think it's just that F1 doesn't have the popularity here to support a serious push for American talent in F1. Look at Scott Speed, he didn't do all that well in F1, but at least he was in there, competing at the highest level. But after only a couple seasons he comes home to race NASCAR, whether that was his decision or Red Bulls I don't know. But if it was his decision I completely understand it, to be home with your family, instead of halfway around the world for 2/3rds of the year, makes sense to me.
My hope is that with a race coming back to the US, F1 will gain some popularity here, which needs to happen before we see a real American contender I think.
I agree with RTHolmes too, specifically about the Fins. They grow up driving on crazy roads from the time they are 12, driving is just in their blood. Mikka Hakkinnen will always be one of my favorite drivers.