Answer:
Because of the nutritious foods that were introduced to the continent.
Step-by-step explanation:
During the 16th - 17cth Centuries, Average African people started to implement healthy diets into their life (with high amount of grains, vegetables, and less processed foods).
This made the natality rate of the people In Africa increased. Even if slavery constantly decrease their population, the amount people that reproduce and have healthy children still outnumbered the amount of people kidnapped by the slave traders.