Jared Diamond's theory in the book Guns, Germs and Steel is that trade, ideas, and technology develop and disperse along routes where agriculture flourished. The basic agricultural package which works from China to England does not work in sub-Saharan Africa. The Bantu people developed one that did work, but it developed a lot later, and there was a sufficient gap between the two to prevent significant commerce, flow of ideas, etc. The result is that Africa was technologically thousands of years behind.

The accelerating technological dynamic that put Europe ahead of everyone else along the Silk Road had a different dynamic. One of whose key ingredients was the lack of any central authority who could suppress discoveries and lines of research for whatever political or religious reasons. And once that turned into a runaway train of progress, well, everyone was backwards compared to them!