Because they are always boring, full of mistakes, and overproduced. Even if they get a good band or singer, the results are boring, and even embarrassing. The producers assume that the audience is all fourth grade students who like pretty colors and shiny crap.
Umm... some people only watch for the Halftime show. With Bruno Mars and the Red Hot Chili Peppers, this year should be one of the better SB halftime performances