There's several reasons why. The less controversial reason for this is that it's simply *really* hard. The Wii Motion+ implements twelve new sensors, and every single game that uses the WM+ uses the sensor data differently! So each game would need specific settings which are impossible to get perfectly. UX for that would be awful, and it would never really work right.
There are other reasons too, but that's probably the only one users care about.
There are other reasons too, but that's probably the only one users care about.