You can say that you have a tight schedule and limited resources, and there is more established talent available than you can accomodate. You can say that the committee is forced to balance their schedule between new talent and established talent, and if that isn't enough, you can tell them that they don't have the name recognition with your audience, because they are younger.

Best, however, is to just say "No." and leave it at that. They've heard it before, and it as more dignity than giving excuses.