Increase amount of war occurred in West Africa. European and American slave traders didn't go into African countryside and kidnap them, they bought slaves from coastal kingdoms. Because they wanted more slaves coastal kingdoms were encouraged to cause more war and raids against their neighboring kingdoms.
The coastal kingdoms became rich and more powerful due to the fact that they were able to get guns, and plenty of other things for slaves.
Slave traid took away millions of Africans in their prime, and this disrupted the cultures off the African nations, and we're less able to progress.