Learn about Aretha Franklin. The reason is because black people developed large Christian networks that helped their communities. Back then, if you were black and wanted to travel to another state, not every town was safe and you couldn't stay at hotels. So they'd call their churches and stay with other members of the church, and get advice through their churches on which towns to avoid. To this day many black people stipp network through their churches, which gave them a lot of safety.

Christianity, interestingly enough, was not strongly forced on enslaved populations in the US. In the early Colonies, the concept that keeping Christians, specifically, as slaves was still a subject that was frowned upon, so many slavers actually took steps to prevent their slaves from converting, which directly led to several modern syncretic faiths as slaves attempted to keep their own traditions alive, while gleaning what they could from the religious standards of society around them.

By the foundation of the US, the Christianization of slaves had become a hot topic, with slavery supporters often coming out against it, either by the creation of a slaver pseudoclerical class which would 'interpret' it for the slaves (ideally in a way that kept them 'in their place') or by the total denial of resources of Christianity to enslaved peoples. By contrast, many abolitionists were strongly Christian, and one of the common illegal activities of abolitionists in the US was teaching slaves how to read, particularly the Bible, and organizing underground churches wherein slaves could operate their own services and lead their own congregations.

Christianity, as such, was seen by slaves largely as less of the enforced, slaver's faith, and more as simply the faith of the land that slavers denied slaves.