In lots of regions Islam was favoured because they locals believed that Islam and their previous Animist beliefs could go together. That is to say Christianity required a full conversion whereas Islam allowed them to hang onto some of their previous customs and beliefs. Obviously there is also the issue of Arabs conquering African lands, and converted African rulers themselves conquering new lands and converting populations.

Source: Birth of the Modern World, 1780 - 1914 by C.A. Bayly