Because if you look at the Archaeological record, we can see developments like pottery, agriculture and metallurgy appear around the same time in West Africa as we do in Western Europe and in the NE part of Sub-Saharan Africa around the time we see it develop in the Balkans. There were also similar population densities up until around 1500 AD. Now it is true that there weren't as many major cities in Sub-Saharan Africa as there were in Europe but that's probably because in the hot, humid climate of Sub-Saharan Africa, there diseases are way more prevalent so it would be best to avoid high population densities. And if you're going to ask how Africa was able to be "so easily dominated":In short, after the Songhai empire collapsed, different factions fighting each other were desperate to obtain weapons so they traded them for slaves captured in war. And (for those of you who say that the African diaspora should blame the continental Africans for selling them into slavery) if they didn't trade slaves for weapons, they wouldn't have arms to fight against other factions who would then sell them into slavery and use the weapons to fight the next faction. Kill or be killed, eat or be eaten, enslave or be enslaved.
The first documentary series is called "Lost Kingdoms of Africa" but a more accurate term would be the forgotten kingdoms because the existence and extent of these civilizations isn't a recent revelation. They've been known about since antiquity but the history has just been ignored.