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One of the most recent forms of European slavery were galley slaves.
I was not aware of this until I read the essay by C.S Lewis's brother Warnie (Warren Hamilton Lewis) in Essays Presented to Charles Williams (which was the original publication of Tolkien's essay/ lecture On Fairy Stories). The essay was later included in Warnie's delightful book The Splendid Century: life in the France of Louis XIV.
Galley slavery was one of the most extreme and brutal forms of slavery - using men as motors until they died.
Warnie explains how expediency (?necessity) drove the French Monarchy into doing this in response - mostly - to the terrible problem of North African piracy in the Mediterranean. In effect, France used slaves (mostly convicts, but also Huguenots) to prevent its own subjects being captured and enslaved.
Barbary pirates could not be defeated by sailing ships, and apparently volunteer oarsmen were tried and were unforthcoming, or found wanting.
A similar dilemma was faced in the West Indies where African slaves were used in agriculture. When slavery was abolished in the British Empire the plantation owners could no longer survive economically and were compelled to introduce indentured labourers from India (a system rather like military service: these were contractually time-limited slaves who were freed after a certain number of years. After which they often went on to adopt middle class jobs.)
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The point is that - for a given population and a given task - some things are possible with slaves that are not possible without them. Hence there is an intrinsic tendency for slavery to return except if it is continually and actively suppressed; as has been obvious around the world over the past several decades.
Once the will to eliminate slavery at any cost had dissipated, and once that the wish to eliminate slavery was no longer backed-up by overwhelming force (i.e. the British Empire collapsed or was dismantled) - then naturally the institution of slavery returned wherever it was expedient (i.e. in agriculture based, pre-industrial societies).
Instead of eliminating slavery, the Western powers now support it (albeit indirectly). So obviously there is a lot more of it.
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