The Spanish, Portuguese, British, and French all penetrated deep into North and South America by the end of the 18th century. Yet the interior of Africa remained relative unknown despite the continents proximity to the colonizing powers. Why is that? Why were the Europeans initially more interested in the Americas than Africa?
Bartolomieu Dias reached the Cape of Good Hope in 1488, demonstrating that from Lisbon to the tip of Africa was a humongous voyage in a tiny caravel. And if you wanted to get to the Indies, you still had an immense body of water to cross. And as Sir_Spaniard points out, disease in Africa was a serious problem (although as soon as the slave trade got going, the Caribbean had every communicable disease found in Africa).As an alternative to the daunting prospect of exploring or exploiting routes around Africa, one could try going west instead, as Columbus and subsequent voyagers did. And here, I believe, is a primary answer to your question: there's a roughly triangular counterclockwise wind and current circulation in the North Atlantic. Go south from Iberia, and you encounter easterly winds out of Africa that take you all the way to the Caribbean (same route hurricanes follow). When you (and your galleon full of gold) want to come back, you can follow the Gulf Stream until you pick up westerlies that will return you to Europe. This incident of geography greatly facilitated exploration and trade development.
Harland's Seamanship in the Age of Sail gives a comprehensive look at this, and I think Sobel's bestseller Longitude also goes into it, but frankly the historical fiction (Patrick O'Brien, Alexander Kent, C.S. Forester) is more fun.
The simplest answer I can give, is that prior to steam ships that Africa had a population immune to the disease Europeans brought while having enough that new comers had a difficulty handling. One such example was African sleeping sickness which affected horses making overland logistics difficult before the use of steam ships to navigate African rivers.