It's a matter of scale, really. Leno's ratings at 10 aren't far off his ratings at 11:30, but his ratings at 11:30 were far, far less than the numbers NBC has traditionally pulled at 10. Affiliates depend on the 10 p.m. show to lead into the money-making local news, and they're extremely unhappy that Jay has been dragging them down. Particularly alarming is that Leno's show has been attracting fewer viewers than some of the shows on cable nets such as Bravo. That's shocking for one of the big three networks.
NBC knew Leno would never draw the numbers that a scripted program would, but they thought he'd do better than he has, and that the cost savings would balance out the loss of viewers. But the whole plan has backfired. As it stands, Leno's ratings are in the tank, local affiliates' 11 p.m. news ratings are in the tank, and the Tonight Show ratings are in the tank. Things have turned out about as poorly as they could have.