Short answer please and this was during the 1950's.
This is my deduction-- not any sure fact, but something to think about. Remember the 1950s were a very conservative decade. The majority of Americans were conformists, and deeply entrenched in religion, the Bible, etc. In the Old Testament, the slave trade was approved and there is mention of slaves and slavery throughout. I remember hearing people state that since this is in the Bible, it is supposed to be, and with their religious and political conservatism, it was deeply embedded in their beliefs.