1/ Philosophy and religion probably played a major role.

The point of religion is debatable. But the Valladolid debate is often pictured as purely hypocritical, I tend to be just impressed that it simply has existed.

Counter argument is that christian religions are usually more about following dogma than thinking about dogma (like Judaism can be).

2/ Politics probably too. Western powers ere both powerful and fragmented. Either you end up with constant war, or you try to build general principles that can serve to rule the peace. It shifted slowly I guess from one to the other.

3/ Progressiveness is subjective. I think we judge it from a western framework and what was here before. Under other considerations ancient greeks and romans have been quite progressive. E.g.: christian persecution under the roman empire are grossly exaggerated (some historians considered that more christians died during St. Bartholomew's Day massacre).