The Kingdom of Kongo became extremely wealthy and powerful through establishing the Atlantic slave trade with the Portuguese Empire. Its first explorers established relations with Kongo in 1483, and additional migrants gradually began building coastal settlements and trading posts.

The banning of the slave trade in the 19th century severely disrupted Kongo's undiversified economic system.

https://en.wikipedia.org/wiki/Angola

Banning the slave trade made Africans poor and they blame slavery for poverty