For the same reasons White people follow those religions.
1. Fear / Ignorance
2. Parental/family influence
3. Lack of knowledge of other religions
4. Spiritual fulfillment in Islam or Christianity
When I was looking for a religion, I did investigate Santeria, Yoruba and Vodun. While I liked the music and dancing, and how the loa / orishas would visit the people during the services, I didn't join because it reminded me too much of my mother's Oneness Apostolic Pentecostalism. I dont like going into trances and losing control in front of people. I dont like animal sacrifices. Then I looked into my European heritage. A friend introduced me to Vanatru, then I learnt about Asatru.
I'm an Asatruar / Heathen today.
NOTE: I'm thinking the questioner is saying that Islam and Christianity are foreign to Africa. African people had their own religions, (Animism, Shamanism, Akan, Yoruba, Kemetic, etc.) before Christianity and Islam imposed themselves on the people. Neither Christianity nor Islam is for all. Different people see and experience divinity differently. Why should all of humanity follow a Middle-Eastern Semitic pathway?
If you're African-American, questioner, then you know there is a bias / propaganda against African-based religions. People associate them with Satanism or witchcraft. People have been brainwashed for generations against African culture beginning from slavery times. It was part of the indoctrination method to turn Black people into 'docile' slaves.
They punished people who spoke their own languages and practiced their religion. In Catholic countries, the ancestors disguised the orishas / loa with Catholic saints. In the Protestant US, they couldn't do that easily. The subject of punishing people for speaking their native language is a sensitive issue with me, because it didn't only happen with Black people. I'm Black Welsh/Ukrainian and the Welsh were punished by the English for speaking Cymraeg. Nearly the same thing happened in Ukraine. Invaders/enemies will try to destroy cultures of 'defeated' or 'enslaved' people in order to control them.