The British slave trade (which brought over most of the ancestors of today's African Americans) was concentrated along the coast of what is now Ghana, Benin, and Nigeria to the extent that this became known as the "Slave Coast". The Portuguese also got a lot of slaves from the Kongo Kingdom (now in Congo and Angola) and southeastern Africa (Mozambique, southeastern SA), but these slaves mostly went to Portuguese colonies.

Before the Atlantic slave trade really picked up, the Slave Coast was home to a number of kingdoms, states, and cities- let's put this "primitive tribal Africans" thing to rest right now. These states were just as rich and powerful as European states at the time; they only lacked things like gunpowder and writing because unlike Europe that part of West Africa was somewhat disconnected from the larger exchange of ideas and technology in Eurasia.

While West Africans had native forms of slavery and bonded labor, they were much more humane (more accurately, much less inhumane) and practiced to a much lesser extent than plantation slavery in the Americas. However, Slave Coast states were eager to trade with Europeans, and as the Europeans were colonizing the Americas what they needed most was labor. The Slave Coast states provided slaves to fulfill that need. Demand was enormous and far outstripped supply so these states changed to accommodate it. They developed full-blown slave cultures and increased militarism (to capture slaves).

(I think this is an important point because often you'll see people trying to excuse European powers because it was native Africans who sold them the slaves while, in reality, Europeans were essentially paying them to fight wars and enslave people. It doesn't excuse the African leaders who participated, but it's necessary to have perspective.)

So, long answer short: African Americans are largely descended from people living in the southern part of West Africa because of the presence of states that participated in the slave trade.