The northeast states are built on a legacy of making life hard for anyone who didn't hail from the right country or worship in just the right way. The west coast treated Asians like crap. The plains states had to kick the natives out and that wasn't pretty.

It's harder to fit that crap into something short and sweet enough for history textbooks so you don't hear about it whereas simple skin tone based discrimination in the south is easy to make a bullet point out of so everyone knows about it.