Mwarabu feared Maasai - so trade hinterland never floorished. Maasai abhorred slavery of any form - and so did many east africa tribes. I think the same was true in South Sudan - Nilotes abhor slavery - the Arabs could never penetrate tribes that did like slavery - Oromo/Gallas (free people) also refused to subjugated by Ethiopian or arabs. Somalis converted to Islam and got near equal treatment with Arabs.
These mostly Nilotic societies were equalitarian - there were true democracies without single leader - where all people were near equal and those better had to show their smartness, leadership or ingenuity - more like American gun ownerhsip ambition to get true form of democracy.
West African, Uganda, Congo and South Africa were led by Kingdoms - who had slaves already - and had no problem selling their people or their ivories- for piece of Arab clothes and goodies.
Yep. Contact with Islam seems to explain some of it. For some reason the culture did not spread into the hinterland in East Africa until late.