Christians didn't proselytise in pre-colonial Africa whilst Muslims did.

By around the year 800, most North African Berber tribesmen followed Islam and they had long used the desert routes into West Africa, through which they brought Islam to those people.

Christians, other than in Ethiopia, didn't have an outpost in Africa before the colonial period to work through. Furthermore, even when did colonise parts of Africa, such as the Canary Islands and the slave colonies, they were not bothered about spreading their religion to the indigenous people. There were many debates at this point (the 16th and 17th centuries) about whether Black people could receive the word of God at all (see Bartolome de Las Casas and Sepulveda). Therefore Christianity only makes significant inroads during the age of high imperialism and Protestant missionaries from America in the 19th century.