Honestly I have no good theory.  Here are some rejected ones:

  1. They are the only ones sufficiently lacking in self-respect.
  2. Since ads drive everything what matters is the audience still watching at halftime.  By that time the geezers have already drunk enough to be glued to their sofas, but not enough yet to be asleep.
  3. Only Pete Townsend’s generation knows how to count to XLIV.
  4. It’s a kind of best-of the has-beens competition.  A shadow rock-and-roll hall of fame.
  5. You can hide their spectacles and tell them its Carnegie Hall.
  6. Minimizes the, ehem, fallout from wardrobe malfunctions.