The short answer is that there weren't major attempts to colonize Africa at the time. Europeans were too vulnerable to African diseases, and it was not uncommon to see mortality rates as high as 90 percent in European settlements along the coast of West Africa. This made it almost impossible for Europeans to venture into the interior and extract resources themselves.

Diseases in the Americas could be deadly too, but it was more manageable. Africans were also less susceptible to Old World diseases than the Native Americans were. This made colonization easier, but this does not mean all the Native Americans just died off either. Colonization was a gruelling and deadly process any way you look at it, it just so happened that the Americas presented better opportunities that Europeans took advantage of. The availability of land and access to lucrative natural resources made American colonization more alluring. It's important to recognize that not every colonist set out wanting to forge a global empire. Most were just looking for an opportunity to improve their own lives. Over time we tend to think of colonization as a broad and uniform process, but it was actually very complex. Many European colonization efforts in the Americas flat out failed for a variety of reasons.

May I ask the name of the documentary?