heh. the biggest driver of anti-slavery in the u.s. was high-paid mechanics' farmers' and laborers' resistance to slave labor driving down their wages. in calling for "free states" they primarily meant "free of blacks".
one of the biggest arguments in favor of american black slave labor was that only blacks could tolerate working in the climates best suited to certain crops, and that without slave labor those crops never could have been produced in any useful quantity. applying this to traveller one may readily observe that given many planets' wide range of environmental conditions there are many niche but large settings where only "certain people" can be profitably, buit very profitably, employed. extending the "cash crop" analogy one readily observes that mining work is always unpleasant and often has been assigned to "prisoners" of one stripe or another. and voluntary slave labor may be induced via drug addiction - consider the movie "outland" and the impressment of sailors who were given a ration of free rum every day as part of their salary.
Indeed, I've read complaints made by Southern mechanics who called for limits on the use of slave labor in the trades.
So much for slave labor not being competitive.
The climate argument was always a bit silly in North America, although I imagine when you were comparing Irish slaves and West African slaves in Barbados, it held up a bit more.
As I understand it, the Irish numbers dropped for the simple reason that the English had finally pacified the island, killing or exiling maybe one in ten of its people. The landlord class needed a lot of the rest as farm workers, afterward.
Whereas the various pre colonial West African kingdoms and tribes were frequently at war with one another, so they provided a ready and continuous flow of slaves.
Warfare as a source of slaves is something one finds in many cultures and in many periods of history. I imagine everybody knows this. I hope I don't seem like I am lecturing. Rather, I am looking for possible adventure and setting ideas for Traveller. Any races or worlds that normally capture and enslave enemies?
Maybe a mercenary crew ends up captured and enslaved, and the guys who escaped decide to go back and free their comrades?
A world government that has just gone through a civil war decides to sell supporters of the defeated faction off world, and the PCs' Patron hires the party to track down a friend or relative before auction?
Hunting runaway slaves may be a good adventure possibility for PCs, too.