This. Other parts of Africa had not gotten their own shaka zulus to group the surrounding tribes under one unified culture. People were still divided in semi autonomous clusters and identified to a clan/group. On top of that....meeting euros did not reinforce any sense of unity if anthing it further alienated respective tribes to further identify to their clans. Some thought they could align with euros and defeat other tribes. Instead euros had a bigger plan in motion which ushered in trans atlantic slavery on a mass scale.Because African tribes weren't really unified. They didn't see the continent of Africa as a thing to protect. Only their own land/tribe. So it wasn't difficult for a unified Europe to take over piece by piece.
Slavery has always existed throughout the world but not under the guise of race.