As someone who lives in the USA, I’d guess it’s because of multiple factors:

(1) USA is one of the most diverse places in the world. So when you have so many people with diverse cultural, linguistic, and religious backgrounds, this naturally leads to tension.

(2) Equality and freedom are a strong part of the American identity, so people vigorously fight and protest for whatever their version of that is… and our strong freedom of speech laws and non-tyrannical government let’s them do that.

(3) Compared to other ways people can be diverse, race is usually pretty easy to identify.

(4) Race-related topics are frequently shoved in our faces by the media and company/school diversity initiatives.