What do you think are the reasons that humanity has largely started to treat their prisoners and foes in ways that do not involve horrors such as torture?

The history of this is relatively tangled. In medieval European warfare, knights were routinely taken prisoner, and were treated well. Often they could go home under parole -- basically a promise not to take up arms again until formally exchanged. (Barbara Tuchman has a long discussion of this in A Distant Mirror). Upper-class combatants were basically a transnational military caste who extended professional courtesy to each other.This only applies to nobility of course. Peasant combatants could be slaughtered out of hand.

Likewise today, different prisoners are treated differently. We have some pretty dreadful prisons in the United States.

In summary, there's an incredible diversity of how prisoners are treated and there always has been. There may have been a general rise in standards, but it hasn't been systematic and I don't think it's been monotonic.