Slavery was always widespread; it was Westerners who saw the moral inconsistency between their self-image and owning slaves, and then fought wars with one another and others to end it globally.
The journey from serfdom to modern human rights was a long one, but it had a clear upward trend.
Even so there were clear steps in the right direction.
Monarchs grew steadily more circumscribed from a de jure point of view. Eventually they became constitutional monarchs, then mere figureheads. Their de facto power also went from absolute steadily downward.
The same trend happened with Christianity: at one time the Pope was so powerful, he broke up the Holy Roman Empire. But there were many reformations, revolutions, Protestantism.
There's a clear trend of decentralization across the entire West, spanning a thousand years - because of individuals insisting on what we now call their inalienable human rights.
In addition, SS requires a budget so is more open to controversy. Respecting individuals is free; violating their rights requires a budget as well.