Because Africa has so many heathens who don't know about Christ and everything He does for people and they don't think it's fair that this allows them to enjoy their lives...
They concentrate on areas where people don't know that much about the church like a bunch of stalkers. There are resources to be had and people don't know they are being used.
Africans of various regions, but mostly West Africa and Sub-Saharan countries, are incredibly religious. They love being invited to other churches. They love joining them, as well.This has been great for Mormon growth. But most of the other churches have value-added -- medicine, education, etc. Mormons got nothing. Just guilt and shaming, as usual. But they have a card up their sleeves -- magical thinking! Like magical underwear that protects them from witches and other goblins and such. The underwear, for example, is perfect for making sure that, if a person buys a "fetish" from a medicine man or sorcerer to shrink one's penis, one is protected. Senior Mormon missionaries and mission presidents have no clue about how many members still believe in sorcery, and have no appreciation for the level sex plays in African cultures. But the Africans of various countries sure pad the membership roles, and that's the real interest.
It assuages the colonial guilt of rich white kids. It also allows them to pretend that they love black people and want to groom them instead of letting them develop stable societies of their own.