Who is the only American driver to win the F1 championship?

 It wasn't Mario Andretti

Why do US drivers struggle in F1?

If you consider this question, you will probably get to the wrong answer, the most common answer would be Mario Andretti, but that is incorrect. Andretti was a naturalised American, but he was born in Motovun, Croatio.

The correct answer is Phil Hill, the answer depends on the place where the driver was born, not if he became naturalised.

US drivers struggle in Europe because the tracks are more technically demanding than the long tracks in the USA. This is also the reason drivers from Europe do well in NASCAR. They are used to the more demanding tracks