I've been blessed to study with many people who hold a variety of religious and non religious beliefs. I have a friend, Wilfredo, who converted to Islam; another friend, Andrew, who is devout to his Jewish heritage and religion. I've studied with Christians, Atheists, Agnostics, and a number of people who have lesser known religious beliefs. What became clear to me was the way faith and reason functioned inside these systems of belief.
The three dominant western religions all have scriptures that can be used for the justification of slavery. When you add religious approval to in-group out-group bias, you have a moral slip n slide in which the use of humans as tools for production is acceptable.
This explains the subjugation of people, but does nothing to deal with the question of someone being a "born slave". I'm comfortable knowing there's no serious conversation about moving our culture back to a place where people, who have a predisposition for servitude, could be easily exploited by individuals or market factors. We should strive for a society in which we actively prevent the exploitation of people, even those who may even want it.