I'd be inclined to question whether having limited knowledge of a wider world than your own neighbourhood would really have been a significant factor in making people capable of the kind of detachment from human values that chattel slavery as developed in the USA required. It could well work completely the other way about.

The people who drew up the American Constitution, and ensured that it left slavery alone, seem to have been quite a sophisticated and kwowledgeable bunch. That kind of sophistication can be quite handy in enabling people to have that sort of detachment.