I don't understand the question. Do you mean, why did trade revenue never reach Africa? Do you mean, why did societies not develop wealth south of the coast of northern Africa? I'm not saying any of these are true, but could you specify the question?

So, based on some of your replies in this thread, what you are specifically asking is why is North Africa not as rich as Mediterranean Europe?

I guess my answer to that is, how far back do you want to go? During the days of Carthage and later Rome, North Africa (at least, modern day Tunisia and parts of Algeria) were very rich indeed. Indeed, during the later stages of the Roman Empire in the west, Africa was one of the key breadbaskets and sources of revenue (and Peter Heather suggests that the loss of this territory to the Vandals in the 5th century was one of the linchpins of the quick collapse of the Empire in the west). There is some belief that the territorial contests in this territory during the 4th, 5th, and 6th centuries essentially helped depopulate the land to such an extent that it lost its former productivity permanently (a similar argument is made for Southern Italy vis a vis the Gothic Wars). However, there is also some evidence that climactic changes have made agriculture in these areas less and less viable; I am not an expert on this particular aspect so perhaps someone else can expand on it.

However, whatever the exact cause, agricultural production declined over the following years. Agricultural production is the lynchpin of wealth in the pre-industrial era, and thus those states with more agricultural production have more people, and often more surplus with which to trade. That said, there is also perhaps a socio-economic argument to make here. Agricultural production also often means a more dispersed wealth base of nobility. In the case of North African cities, there was actually quite a bit of wealth, as these cities became bases for slave trading and piracy throughout the area. However, the wealth became concentrated into relatively few hands, with investment going towards ships at best and bobbles at worst, rather than increased agricultural productivity. In addition, much like the Turks, the North Africans were cut off from the explosion of technology and learning that occurred in Europe for cultural reasons (and the piracy and slaving probably didn't help much there either) which meant they were left out of the growth in wealth as well.