(1) M-16s require much more maintenance than a Kalashnikov or its Soviet-era ilk. Your average African militiaman is simply too poorly educated, untrained and unfamiliar with mechanical devices to really maintain anything complicated.
(2) The primary weapons dealers in Africa are either Chinese, Ukranian or Serbian. Their wares are almost exclusively former Soviet weapons, or their modern counterparts.
(3) Africa is awash in Kalashnikov ammo. It makes military and business sense to keep your men supplied with stuff that can easily be repleneshed locally.
The *Klashkof* [African attempt at pronunciation of *Kalishnikova*] is generally preferred for simplicity and availability of ammunition. But the M16 is sometimes a status symbol, and the Russians found to their horror that some Africans found the M16 to be more desirable because it's finish and the colour of the plastic stocks and foreend was black. More Juju.
Accordingly, the Russians reworked the plum-rustred pistol grips and foreends of early synthetic stocked AKM and AK74 rifles to a black polyvinyl-nylon composition. Hey, whatever the customer wants!