That's because most 'black's encountered the branch organization of the Islam movement, called Black Muslims, in prison. It was a lifeline for them. A discipline. A faith. It promised work for them when they got out. It helped to get them released.
Once out, they are soldiers for the cause. There is no 'out' for them either. Just ask Malcolm X.
It's like a drug you get addicted to, and if you try to withdraw, you die.
So why did Black Moose succeed where the gospel seemed to fail?