Well, let me educate you on this. Christianity did in fact spread through Africa. All along the Mediterranean Sea, over to the eastern side, it spread like wildfire! Even Egypt was once mostly Christian. Also, the Middle East was once mostly Christian.
Now let's turn our attention to the conquests of Islam. This so-called faith knocked out a lot of Christianity from eastern Africa. Look at how Egypt, Ethiopia, Eritrea, Algeria, Libya, and Morocco are now Muslim nations. How do Muslims rule their nations? The faith of Islam is used to determine secular law (which there really isn't any secular law), and Christians were either executed or run underground.
Now, let's go a bit further. Question: What's the largest natural obstacle in Africa? Answer: The Sahara Desert. Is it any wonder why Christianity spread among coastal lines? Not only did the Sahara serve as an almost unconquerable obstacle to the spread of Christianity, it also prevented the migration of culture and technology to the sub-Saharan regions of Africa.
Christianity was already there in Africa when the Muslims took over. There's your reason(s).
Any recommended reading on the subject?