1. Africa has a large land mass but very few natural harbors suitable for modern trade. Africa's land mass is almost three times larger than Europe's but its coastline is 25% shorter.
2. The "fruit tree principle" is very much at work in Africa. Basically, the lack of a harsh climate means it doesn't take a lot of hard work, innovation, and ingenuity to survive.
I’ve spent some time in Jamaica, and often thought that they’ve had little reason to become industrious since they could pick fruit off of trees all over the place and it seldom got cold.
Besides harbors, Sowell points to the small amount of the continent open to navigable rivers.