I suppose it could be a lot of things, but the largest is merely an avoidance of using "black". It's unlikely that anyone descended from slaves has a clear genealogical record tracing back to a nation of origin.
Another is the colonial nature of Africa in the modern era. Aside from the nations of northern Africa, virtually the rest of the continent was or has been sliced and cut up from colonies of Europe and as such do not have the nationalism or culture associated with nations or kingdoms. These kinds of entities DID exist in the past, but not any longer. So if in some far future, your descendants become subjects of foreign powers and the nation known as the United States gets chopped up into several pieces, are you likely to form an allegiance or attachment to your new nation - or the one your parents knew?
Because of this, many nations in Africa cross the boundaries of the nations that exist in the hearts and minds of its citizens. Only the notion of Africa or in some cases, West Africa, have any significance.
This is largely my opinion from my reading. If anyone cares to enlighten me further, I'm all ears.