Is it geographic in nature? I imagine traversing the Sahara desert or boating around Africa to be a difficult task. So, the usual tech and innovation trades that were associated with the silk road weren't in tact?

Maybe it might have to do with my own personal educational background as well, a fault in the USA's Eurocentric education system that hides the true developments of African nations. (Granted, I will lend myself some credit and say I know about how awesome Axum/Ethopia is, at least up until the communists). So, maybe I simply don't know?

Maybe some other elements I'm not aware of?

And just for clarification, this is before European colonization, not during or after which is generally considered FUBAR.