Probably because of diminishing returns. PS4/X1 software doesn't look near as much better to PS360 as PS360 software did to PS2/Xbox.
PS4/X1 software looks "really nice", but the leap is not as big, at least so far.
Thus Wii U making the last gen jump gains more.
That said, I remember a few games that due to artstyle were said to look good each gen. Example, Mario Galaxy on Wii, did not look bad even compared to PS3/360 games. Similarly, Mario 3D world or whatever it's called will look fine next to PS4 games.
Lack of power shows up more in certain demanding genres and certain artstyles. Mario Galaxy on Wii looked nice last gen, but the Conduit (FPS) looked terrible. Similarly, Mario 3D world may look fine on Wii U, but a BF4 port would look horrible.







