I can explain with images, at launch developers on a limited budget were able to port games to the XBox 360 that looked like this:
After developers got more comfortable with the hardware, updated their tools, and started making games from scratch for the platform games started to look like this:
Now, even if the Wii U is significantly more powerful than the XBox 360 or PS3, many developers would be worried that unexpected performance issues of unfamiliar hardware would result in needing to reduce graphical quality to get a decent frame-rate; especially in the wake of the PS3 where you regularly see the frame-rate drop on games like Mass Effect 3 which have (relatively) steady frame-rates on the XBox 360.
The fact that graphical quality is maintained on early generation games that are quick ports of highly optimized HD console games implies that the Wii U is a reasonably powerful system that is easy to work with.
