The book Pathogenesis published this year has a hypothesis -- diseases from the Neanderthals and other hominids who populated the regions outside Africa earlier wiped out the earlier waves of Sapiens.

Until diseases spread back and forth enough that Sapiens had enough immunity to non-Sapien diseases, but not vice versa (due to lower population density or less diverse fauna outside Africa) which wiped out the non-Sapiens populations outside Africa.