Earlier quoted context omitted.
Slavery, throughout history, was generally not seen as desirable. Rather, it was either seen as a necessary evil, upon which a "logical rationale" (read: cognitive dissonance) was built up to justify it, or as a form of punishment. Aristotle actually predicted its end about 2000 years before it happened [1]: --- For if every instrument could accomplish its own work, obeying or anticipating the will of others, like th…
That is an immense leap from "if we could have machines do it, we wouldn't have slaves do it" to seeing slavery as undesirable. Aristotle was a staunch believer in "natural slaves" who didn't have the faculty of rational deliberation and therefore were (and should be) guided by those who do have that faculty. This line of thought can be traced all the way through to the modern day, and obviously well up to abolition.…
But it never remained abolished because of a simple logical problem you run into. The reality of the world - past, present, and future - is that stronger powers dominate weaker powers -- the same reality upon which Aristotle based his cognitive dissonance. And so so long as slavery provided a significant material benefit, powers that embraced slavery would dominate those that did not. And that dominance would inevitably lead to the institution (or reinstitution) of slavery in the weaker powers. The British Empire and its spread of slavery around the globe is but one of many examples.
So you saw this regular flip flopping. One ideologically minded leader would end slavery, only for it to come back later. What changed with the industrial revolution is that the latest end of slavery no longer had any particularly negative consequences, instead we saw the exact opposite. Countries that abolished slavery, which was primarily a rural/plantation based phenomena, actually started to grow exceptionally rapidly on the back of the emerging systems of industrial wage labor, while rural and plantation style production became less and less economically relevant. Slavery had become obsolete.
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There's some interesting parallels with slavery and modern day conscriptions. A country locks up its borders, prevents people from leaving, starts forcibly conscripting them into the military, gives them a gun, and sends them off to die. This is utterly barbaric, and most people would agree with such. Yet it remains a thing, and will remain a thing for the foreseeable future, for the exact same reason that slavery was perpetuated.
Countries that turn to conscription will be more powerful than those that don't. When this reality becomes no longer true (perhaps due to war becoming more mechanical in nature) we'll certainly finally abolish this barbaric behavior, and then claim it's due to some 'greater moral understanding', as if people alive today can't see with their own two eyes what an unnatural and abhorrent behavior this is. Of course we can! But trying to permanently stop something that's a significant means to power is like trying to stop a train by walking in front of it.
[1] - https://en.wikipedia.org/wiki/Timeline_of_abolition_of_slave...