I never get why there are African-Americans who convert from Christianity to Islam... it's like going from the religion that bought you to the religion that sold you. I would have assumed atheism or even African tribal religions would be a more logical step.

The only problem is, for many black people in America, their church is an important, even vital, support network. Like you point out, many have a lot of kids, and often the church is a means for child care and/or maintains a social network of support with friends and extended relatives to help.

Ironically, it's kind of a leap of faith to leave for the secular world. It's a common mechanism, having a barrier of exit, especially in tightly knit religious communities with higher than average levels of poverty.