The answer is fairly simple.. although the details can be quite complex.

First, the US is so much larger and diverse. People of different backgrounds settled different places. With several thousand years of history and until recently 'rulers', it may be hard for you to comprehend a 'new' nation made up of settlers trying to invent a common relationship.

Second, when we officially became independent, the various states were far apart and had only a little awareness of what the others were like... so "states rights" was written into the Constitution, giving a huge amount of discretion to each one as to how they were governed. The South was largely rural, and became dependent on slavery to do the farming, etc.
Then, as railroads and telegraph brought the states 'closer', slavery became an issue and we fought that long Civil War over whether there should BE a 'union'. The south lost, but many never quite got over either losing or having to accept former slaves as equals. Thus, attitudes of racism, while seen everywhere, were more pervasive in the south. (You in the UK are not immune to the basic issues of racism.. you just don't have laws which implicitly support it in different areas.)

Entire books have been written trying to explain all the changes and sociologists debate endlessly why racial stuff persists. Short answer... we are dealing with humans in a very complex demographic situation...and we have Mexico on our southern border, which further complicates the current situation.