I don't think it's in spite of racism and dominance, but rather because of it. Christianity tells people that it doesn't matter what their race or economic status is, they can go to heaven if they have faith. When people are in poverty or discriminated against it probably feels good that their position is irrelevant to the creator of the universe and they'll be in a better place when it all ends. It also gives them a tighter sense of community.
Also, lest we forget, there is very little remaining of the native West African religions or culture. This is for several reasons, nearly all of which boil down to Europeans being horrible racist fucks for a very long time. Specifically:
1. When you're forcibly removed from your home and sent to an entirely different continent, housed with people who don't share your language, culture, or gods, and sold to people who don't share any of those things
or skin color, it's really hard to participate in anything that requires a group. It's also hard to participate in things requiring a trained priest or priestess if you aren't one.
2. When you're working from can to can't every day but Sunday, you don't exactly have much time to pass on your religious or cultural heritage to your children. This goes double if you're expected to attend church on Sunday with the people who own you and control your life.
3. It wouldn't surprise me (although it would be pretty horrible) if slaveowners and overseers beat the "superstitious heathenry" out of their slaves if the latter were ever seen practicing their native religions. It was also easy to blame a slave for casting hoodoo curses on your household or livestock (see also Tituba, the slave woman who was essentially the first accused witch in Salem, MA).
4. The colonization of Africa in the 19th century basically did all of the same things, in new and horrible ways.
5. Not all of the West African peoples still practiced their native religions when the Europeans started enslaving them; Islam made it to Africa within about 200 years of Mohammed's death, and spread through pretty much all of the Sahara region.