The boys main goal was not to become famous. They had not anticipated the fame. They boys just wanted to sing gospel music. No matter how many attempts to pervert gospel music, the boys stood true to it. They did not include other genres such as pop because of their love for gospel. In an interview with NPR, Lulu Garcia-Navarro and Jimmy Carter were speaking on this:
Lulu Garcia-Navarro: Mr. Carter, when you released your debut single in 1948, did you ever think you’d still be recording all these years later?
Jimmy Carter: I had no idea. And I always tell the people who ask me: When The Blind Boys started out, we weren’t looking for any accolades, awards or nothing — we just wanted to get out there and sing gospel music. But since the accolades came, we were glad to get them! [Laughs.]
I want to hear from you about this; let’s talk about “Stay On The Gospel Side,” which speaks to that sentiment. Mr. Carter, why did you want to stay on the gospel side? You could have made it big in pop or rock or blues.
Carter: Well, we could have. But you know, when we started out, we made a pledge. We said, “No matter what, we were not going to deviate from gospel music.” This was what we came out here to do and this is what we are going to do. We had a lot of people who crossed over, and in fact when Sam Cooke crossed over, we were right there at the same time in the same studio. And they offered us the same deal they offered him. But we turned it down, and I’m glad we did.
Source: https://www.npr.org/artists/17085871/blind-boys-of-alabama