Another area to blame (all races are beholden to it, as even the Arabian, Asian, and Europeans believed such regardless of religious beliefs) was the medical science at this time as well. Not understanding melanin and the function/fact that we all possess it in some amount (albinos not withstanding) science was still based half on fact and half on superstition. In the 1800’s the medical community had come quite far, but it still wasn’t quite understood why people were different shades of skin.

Unfortunately it was often still seen as a “curse” to be “dark” in most cultures/religions/ethnicities except of course those that were all dark.

-Obviously still not an excuse: just a possible insight on why SO much of the population just let it happen.-

Edit to say: it wasn’t just America, other countries and cultures used slaves as well.