Disgrace: Slavery

It is certainly easier to enslave another human being if you can't hear their humanity elucidated in language.

African langauges groups were much smaller, so the stories of slaving could not have spread very thoroughly. By thorough here I mean that sure there were translators, but penetration of these ideas could only be along those lines, and not in casual person-to-person meetings on the borders. Witnesses could only travel so far to tell their tale, limited by the geographic range of their own multi-lingual talents.