Various reasons.
1. Most are weaker than ps3/360 except for racing games and 2D games
2. I own a PC and 360 and PS3 so have other options to buy games
3. I prefer the 360 and PS3 controllers for most types of games
4. My AV amp only supports optical or coaxial input so no surround sound if I buy the wii u version
5. wii u games are often slower loading which is annoying
6. If I'm buying a downloadable game I prefer to use steam or psn
7. wii u versions seem to commonly miss out some features
8. No analogue triggers on gamepad which benefit many games
9. short battery life of gamepad means often more convenient to play on ps3, pc or 360
10. some wii u games become unplayable when there is a lot of action on screen and the frame rate drops considerably more than 360 or ps3
