Because the JLS appeal is based on their physical attraction - which aligns with my point.
No-one goes to a Van Morrison gig to drool over Van's handsome chiselled features and Adonis-eque physique - that's not where his appeal lies - as i said, appeal is about demographics, which matter, or not, depending on the arist.
Van Morrison would write and record if he never set foot on a stage again - JLS could appear on stage and never record (I doubt that they write) ever again - that's the difference.