Well, gospel piano player like to play in flat or sharp keys because that is what people mostly sing in...when people start feelin The Spirit and decide to sing, the last thing on their minds are to sing in a natural key. Their gonna sing in a tone that they are mostly comfortable with, and when people normally talk they talk kind-of flat or sharp tone, so its only natural for them to sing like that (for better or worth).
Mostly people who are trained to sing well pick a key like A to sing in.
As for playing in the minors, yall talking bout playing jazz or blues here. A lot to Jazz and Blues songs are played in minor keys, because of the Blues scale and penetonic minor scale...which is heavely used in those genres.