On the one hand, it's important to point out that there were about a billion people in 1860 and there are over seven billion now so even with a factor of six reduction in slavery, there's gonna be more slaves now than at any time in history.
On the other hand, I recently finished David Graeber's Debt: The First 5000 years and he made the point that slavery is pretty much the impetus for money. Within small communities, everyone takes care of everyone else. Everyone has context. When two communities interact, assigned values only come into play over conflict - if I give you a horse and you give me three goats and we're both happy, it doesn't matter that a horse is generally worth two goats. The greater the chance of conflict, the harder the values become and the ultimate prices are generally for blood debts - "you killed my husband, you owe my clan eight oxen," for example.
Things really ramp up when you're talking about warfare, whereby reparations are paid wholesale and where the losers must become "value." The best way to accomplish this is to "decontextualize" them, take them away from their families and communities, and sell them into another where they have no context and can therefore be a trade good. The further away, the better. Lots of conflict, very little interaction, and suddenly we've put a price on a human life. While most communities traditionally had no money, they had a going rate for slaves. Graeber's examples stretch out into Victorian England.
Slavery exists wherever two cultures clash and one is greatly superior to another. I once watched a cell-phone video of witches being burned at the stake in Africa - talk about culture clash (and debasement of human life). It's that delta between the dominant culture and the subservient one that allows human trafficking to happen... and we need to stop thinking of "slavery" in terms of "chained to a cart and picking cotton. The father-of-two from the Philippines that hops a flight to Dubai to send money home only to find his passport held by a ruthless Emirati is just as enslaved as Nat Turner was. In fact, Graeber makes the point that many African slaves shipped to the Americas were debt slaves captured by unscrupulous merchants using schemes drearily similar to payday loan shacks.