Slavery has existed for many years, Many plantation owners used slaves to get the work done faster. Plus plantations couldn't survive without slave labor.
I believe the answer is: Ethnocentricity.
ethnocentricity refers to the act of seeing our own culture as inherently superior compared to another culture. This often make people seeing the norms or tradition that exist in other culture by using our own as a standard. The more their culture deviate from ours, the more inferior we think their culture to be.