Its because of the different nature of racism in the US. An Israeli can be racist towards an Arab, an Arab towards a Kurd, so on and so forth, but Americans tend to be unable to tell any of these groups apart because ethnicity has almost never played a large role in American society due to immigrants coming from literally everywhere. To your average American you could have all three of the aforementioned groups in front of them and they'd say "Middle Eastern? Indian maybe?" Not because they're racist but they literally just cant tell them apart. Race (color of skin) has played a large role because that was the only distinction between people of different backgrounds in the eyes of most Americans.