Jerrod Diamond does a very good job of explaining that in his Pulitzer Prize winning book, Guns, Germs and Steel. The Americas have vast navigable rivers and they have prairies that are suitable for wheat, oats, barley, rye, corn, etc whereas it is much more difficult to grow crops large scale in Africa. Once the crops are grown, we have more rivers that are useful for transporting grain than the rest of the entire world combined. The indigenous tribes of the Americas were absolutely devastated by disease and displacement by Europeans. Diseases such as malaria made that impossible in Africa.
It is idiotic to suggest Columbus or other explorers purposefully spread disease as one answer suggested. His answers are usually good but that part was annoying. It is fairly basic history that shows they didn't even know what caused the disease back then. Even by the time of founding fathers, they didn't know, let alone hundreds of years before that. How could they then be blamed for it. Anyway, they spread the disease but did not do it on purpose. It apparently makes some people feel better if they can make victims out of other races.