"slavery is wrong because it accumulate wealthy on the hands of few, and keep many on misery forever, making it hard to collect taxes from the healthy, sooner or later any government that allows slavery will have a financial collapse, because of deflation(money losing value)"
That was the economic / logic reason why it ended, there is not much difference from nowdays depending on the place, based on wealth distribution at least, still hard to collect taxes from the rich, and we still have people that live on misery forever, it was a good thing ending slavery, but we did a bad job on replacing its model.
I want to see how slavery is implemented on this game like world, with skills and all, let's see if this will be different from other fantasy mangas