I do not usually post but I found this question interesting, as it pertains to my RL field of study. The answer IMHO is the effect of ethnocentric nationalism. In real world to have ethnocentric nationalism you often need key ingredients; shared unresolved historical trauma, race, and ideology. At the end of the Great Hyperspace War, the conflict that led the now Sith Empire to be created, the then Chancellor of the republic ordered the whole sale extermination of the Sith. Men, women, children, soldiers, civilians and force users alike. The refugees banded together under the Emperor Vitiate were unified by their racial ancestry i.e human and Sith species, shared trauma, and dark side ideology. The racism you see is direct result of this a sociological weapon the emperor used to forge a empire that would never forgive the republic and create a us vs them mentality that is to the very core of it's people.