Check out Plato's cave.
These people are told from an early age what to believe and when you lead them out of the cave (ei enlightenment with atheism) they would rather go back into the "cave" of religion and believe a simple lie.
-
Well I wouldn't call atheism enlightenment rather just not being a gullible idiot. The reason people still believe in something like religion is because it is forced into them as children and like any cult it can take a while to deprogram them.