Ibn Battuta: Travels in Asia and Africa, history homework help

For this last week we are looking at the development of civilizations in Africa and the Americas during the Middle Ages.  During this period of time, Islam, originally found in North Africa along the Mediterrainean coast, gradually spread southward into Central Africa, south of the Sahara Desert, from the Red Sea to the Atlantic Ocean.  What was the main reason from the fairly and rapid spread of Islam through Central Africa?