The truly remarkable thing, if one consults the historical record, is that slavery has been eliminated—or effectively eliminated—many times in human history.
In Europe, for instance, the institution largely vanished in the centuries following the collapse of the Roman empire—an historical achievement rarely recognized by those of us used to referring to these events as the beginning of “the Dark Ages.”
No one is quite sure how it happened. Most agree that the spread of Christianity must have had something to do with it, but that can’t have been the direct cause, since the Church itself was never explicitly opposed to the institution and in many cases defended it.
Instead, the abolition appears to have happened despite the attitudes of both the intellectuals and the political authorities of the time. Yet it did happen, and it had lasting effects.
On the popular level, slavery remained so universally detested that even a thousand years later, when European merchants started trying to revive the trade, they discovered that their compatriots would not countenance slaveholding in their own countries—one reason why planters were eventually obliged to acquire their slaves in Africa and set up plantations in the New World.
It is one of the great ironies of history that modern racism—probably the single greatest evil of our last two centuries—had to be invented largely because Europeans continued to refuse to listen to the arguments of the intellectuals and jurists and did not accept that anyone they believed to be a full and equal human being could ever be justifiably enslaved.