For example of a different cause: Nathan & I have written previously on here on a sociological “trend” of racism shifting to the upper South, particularly Appalachia, and then increasing in the North & West of the country relative to the South. It seems as though at least part of the reason for this is the larger black population in the South relative to the rest of the country.
I think it's the inverse. You're seeing "more racist" attitudes in the Northeast and Midwest because they, like the historical South, are becoming more non-White. This is especially the case in "legacy" industrial cities and inner-ring suburbs that were once the epicenter of middle-income, White America. Diversity must exist in a place prior to there being any observable racial conflict, after all, and that diversity is something Yankee parts of the country largely missed out on historically.
North Korea is super racist(like they ban interracial marriage) and there are no known minorities of any kind there.