Generaly we have quite a lot of sources on north african history. Information of sub-saharan Africa is on the other hand less common. One reason is the lack of written sources. For a long time sub-saharan African's did not leave substantial amounts of written records and the ones we have often originate with arabic merchants.

Secondly even though sub-saharan Africa did have contact with the islamic world it remained relativly distant from major connections for much of it's early history before they started taking a larger part in world trade such as the Swahili did. This means that there weren't a lot of non-africans to write about them either.

One might also argue that cultural bias against africans have led historians to chop them up to simply savages without any important history but that is of course controversial.

Edit: Changed "no written records" to "substantial amounts of written records" for more accurate phrasing.