Satinavian said:
But for the most part, slavery is not economically viable. It turns a profit only under very specific conditions. That is usually the main reason for both rise and end of slavery.
Mainly this.
Despite the slobbish, lazy caricatures we often portray slave owners to be, the truth is that they were far more actively militant as far as cattle-slavery is concerned, slaves are people still and have their own wants and desires, even if brought up as a slave, and require constant supervision and maintenance in order to stay productive or even compliant, their lack of self-reliance makes additional logistics necessary.
Looking at it mathematically, you have to keep in mind that to make slavery viable, the productivity of each slave has to surpass the cost of food required to feed them, the timber to shelter them, the salary of the watchmen and the pay of the slavers that caught them, additionally, the labour they are required to do may be dangerous and add additional costs for replacing lost workers.
The problem of unrest is a particular issue, being human beings, these slaves would still be intensely social among each other in their "off-time", allowing them to collaborate, this kind of support may cut SOME cost for the owner, but what happens when a slave is hurt during work that cripples them for life?
The easy, most callous answer would be to dispose of them, but you'd do well to remember that they have friends among the others, they are socially connected, and to enact such cruelties would spark more unrest than already present, sustained injustices gets you a revolt on your hands, causing untold amounts of damage in both material and human property.
From the cold perspective of the master, slavery may be much more trouble than it's worth.