Is there any research about why Sub-Saharan Africa doesn't have Neanderthal DNA?

Is the argument that the tribe of humans from Africa was good at repelling outside invaders, but themselves expanded outwards and assimilated (and then outnumbered) the other populations, or something else?

It just seems a bit bizarre given that all humans elsewhere have relatively similar amounts (but quite a low amount) of Neanderthal DNA, which seems to suggest a reasonable amount of migration, interaction and interbreeding between populations everywhere except Africa.