When Rome had an empire they colonized everything around them: France, Spain, Algeria, Lybia, etc.
After the fall, the power dynamic shifted. How was it decided where the power went? Northern African countries are still Mediterranean just like European ones. Why did European countries on the Medditerranean, like France, become so strong -- but Northern African countries like Algeria just became France #2? Or did it come into power by its own accord and I am just historically ignorant?
To use your example, more than 1400 years passed between the sack of Rome and the French colonization of Algeria. During that time, Algeria was part of powerful caliphates and the Ottoman Empire. In the 15th-17th centuries, Algiera was home to privateers who would capture European trading ships and raid European towns to take slaves - even abducting the entire population of an Irish town. In that last period one can imagine your question being asked in the other direction - "Why did power shift South of the Mediterranean?"
At least in the U.S., where I live, we don't learn a lot about African history and many people know only stereotypes of the continent as filled with poor, uneducated savages or as the helpless victims of colonization. However, a little reading shows one that Africa has been home to kingdoms and empires as grand and powerful as any in Europe.