History is largely outright violence and violent exploitation, and the early growth in complexity in the ancient world is marked by the move from the former to the latter. Primitive states massacred their enemies, their leaders unable to apply force in any more sophisticated fashion. It required more complex states to produce the ability to effectively enslave defeated foes in large numbers instead, but once that was achieved states capable of it thrived at the expensive of others.

There is little to chose from between Roman slavery and Medieval serfdom. Long-term colonial economic exploitation had many of the same aspects even where it wasn't outright slavery. The institution of slavery in its broadest sense has proven to be very resilient. It is probably too soon to say whether its comparative absence for a few centuries is a passing thing, or whether it is in fact the symptom of another systematic shift in the sophistication of states. If the latter, then the introduction of modern systems of banking has an interesting parallel course in history. Consider all of the associated means for better extracting wealth from populations without provoking rebellion.

One might put forward an argument that suggests these advances make it more practical for the elite to comparatively peacefully farm the local population rather than farming other populations by making and enslaving enemies. Thus more of the elites do this.

All of which still leaves us with a very long to go yet in order to become a moral species. I don't see it happening without fundamental engineered changes to human nature, and that isn't a near term thing. It is worth remembering that a thin line indeed separates present elites in so-called civilized regions from those who in the past massacred and enslaved. If today's leaders found advantage in it, they would do it.