The way I see it, Africa south of the Sahara has been vulnerable due not only to the instability that followed after the colonial era, but also due to the fact that before the modern era there was never a large unifying power like Rome, and no systems of writing that were passed on. I don't think this was due to anything like 'race' mind you, but rather, the geography and climate and the types of cultures that developed there as a result. It made Africa 'easy pickings' as it were, thus leading to the current situation today.