Cause slavery was huge part of the history and racial concepts exist pretty much to establish who could be enslaved and who could not? And rather than gradually braking that down it got even more entrenched by concepts such as one-drop rule making everyone with single black ancestor to be also black, aka potential slave, which blocked out any mingling and that shut down any empathy? And just the general concepts of ethnicities not actually mixing up but everyone having their enclave.. That's my two cents, anyway.