"Adopt" is not a word that corresponds perfectly to their decision to take up Christianity in America. They "adopted" Christianity in the same way that they "adopted" brutal labor! More specific to your question and setting jokes aside, their enslavers sought to control every aspect of their life. Furthermore, planters of the Deep South considered themselves superior for recognizing their own superiority and drilled it into the heads of their slaves that they were inferior. Specifically, they believed that every human is born into a state of subjugation and that everyone needs a master of sorts, that it is "the order of nature and of God that the beings of superior faculties and knowledge, and superior power, should control and dispose of those who are inferior." (William Harper) Contrary to the Founding Fathers, Alexander Stephens of Georgia said "the assumption of the equality of all races is fundamentally wrong. [The Confederacy] rests upon the great truth that that the negro is not equal to the white man; that slavery subordinate to the superior race is his natural and normal condition."

In American Nations, (page 203) Colin Woodward writes that "Southern Baptist and Methodist preachers broke with their northern counterparts to endorse slavery on the grounds that Africans were the descendants of Ham, who was condemned in the Bible to be a 'hewer of wood and drawer of water' for his white masters." He also notes a passage from a Presbyterian minister Rev. Fred A Ross: "Man south of the Equator-in Asia, Australia, Oceanica, America, especially Africa-is inferior to his Northern brother...Slavery is of God, and [should] continue for the good of the slave, the good of the master, the good of the whole American family."

So the simple answer as to why they took up Christianity is that they were forced into it. The black reverends were cherry picked to be ones who followed the same teachings as their white counterparts, that slavery is the good and natural state of being, that it worked out in the Bible and in the ancient civilizations that America was attempting to model itself after. If America was truly hoping to be like Greece and Rome, slavery must be part of it.