The author of the book explains it much better and it’s ben a while since I read it - but basically, the psychology of hunter-gatherer types is very different. When imprisoned, or faced with any difficult situation, their inclination is to sit quietly and wait rather than run about, scream, pound on the walls, etc. This is a side effect of not having food supplies always at hand - save your energy until there is an opportunity to solve the situation rather than waste energy in futile displays of frustration. Hunters learn to be patient and wait, even if it takes days, even if they are seriously deprived. It’s a cultural thing. (As I said, look at the scenes of the bushman in jail in Gods Must Be Crazy… based on real life behaviour. The author was a prosecutor in northern Canada, and observed a lot of native behaviour first hand.
Full-time farmers, OTOH, must work all the time for the big payoff at harvest. Crops must be planted, tended, without payoff. Work is not its own immediate reward.
We can get into a debate over whether and when a hunter becomes a farmer, but the fact is the native Americans relied a lot more on hunting than on agriculture in many locales. This was not a country of rolling fiels, but more of untamed forest with small settlements; most areas had not made the leap where the field was more important than the forest.
The natives of a lot of the Americas were hunter-gatherers with small fields where they planted crops to supplement their hunting. There were exceptions - larger civilizations like the Maya, Incas, and Aztecs. But generally, the forest and jungle dwellers were in that intermediate range between hunter and farmer, where for most there was still plenty of game to feed them much of the time.
This was the problem with natives. While this is not a hard and fast rule, the agricultural invaders found that the non-agricultural natives tended to act irrationally when enslaved. I remember reading many years ago, growing up, that one reason for importing slaves from Africa was that the natives often would allow themselves to be whipped to death rather than work as slaves.
As for - who is suited for the work - no doubt dark skin tropical residents were better suited to tropical and near-tropical field work than white-skinned fair-haired northern Europeans, but I’m sure the plantation owners did not care. Africans just came cheaper, in more quantities, with less hassle.
And yes, the original intent of slavery was that anyone could be enslaved. The spoils of war for places like the Roman Empire (and others) was that the conquered locals were enslaved and taken into the empire. This removed the threat of resentful locals of a war-like persuasion, plus answered the question of who would look after the widows and children of the dead.
Similarly, as pointed out above, slavery was the fate of many Europeans captured by Barabary pirates and sold into the middle east.
However, most of these systems allowed some rights to the slave. Brazilian slaves, IIRC, were allowed to buy their freedom, like some Roman slaves. many were allowed to own property and sometimes had certain rights. The southern US system diverged from this more humanistic view, to the idea of slaves are subhumans and mere property with zero rights. Roman slaves could be anyone inside or outside the empire, and were freed and joined common society (as citizens) regularly.