The sales drove off a cliff when games stopped coming out, not because the novelty wore off.Exactly - motion controls were totally new at the time of the wii's release. After a few years when the novelty of the wii mote wore off of the general public the wii's sales dropped of the side of a cliff.
The Wii U also has a novelty controller - but non-gamers arent really interested in a tablet with joysticks/triggers/buttons when the market is already saturated with smartphones and tablets. Add to that the fact you cant take the wii u gamepad with you when you go out and play your wii u games.
Another reason why the normally non-gamer-crowd flocked to the wii wasnt just because of the motion controls but because the 360/ps3 style gamepads seem to complicated - buttons/sticks/triggers everywhere - motion controls with only a couple of buttons are simple and easy for non-gamers.