I agree, but the Greeks and Romans were very active in north Africa. It's the same story, the advances of their civilizations were embraced and utilized by some who came in contact with them, and not by others.
And, north Africa was very much a part of the Greek/Roman world and greatly influenced by those civilizations, until the seventh century when another influence began spreading into those areas from the Arabian Peninsula.
But there was no shortage of opportunities for written languages to take root throughout Africa. They just did not spread very far to the south.
I think they suffered from an Egyptian-descended tradition of an elite literate class (priests and scribes) who were able to resist the “intrusion” of an “alien” alphabet. But that’s just a guess.