Question

what was one of the main reasons the transatlantic slave trade was so harmful to africa

Answer 1

There are main reasons, however among the most obvious ones are:
1. It disrupted African societies, since it introduced foreign goods, that they themselves weren't able to produce. Because of that they became dependent on the more "advanced" countries. Also, the connections between people were disturbed.
2. It took workforce away from Africa
3. Loss of potential, even if only one per million of slaves was a genius, it was still a grave loss