Well, as part of this event he was going to have the Harlem Gospel Choir on stage and singing. Note the tense, was going to. Not anymore. The people at ColorOfChange, who have been chasing of Beck's sponsors on TV, went and talked with the choir about the various comments and actions Beck has been a party to. They had to think on it, and decided to not be a part of the show.
They are saying that it is a matter of not being paid enough.
Yeah, not enough to be seen on stage with the likes of Glenn Beck, I imagine.