Uh ... because the original Wii is running identical hardware to the Gamecube at a little faster rate with more RAM? While the Wii U is going to be running off of totally different hardware? I'm going to venture a guess to the backwards compatibility ... they probably did what Sony did with the PS2 and the original PS3s that had backward compatibility with the PS2 ... shoved just enough of the previous generation's hardware in there for the games to run.
From the Gamecube to the Wii, yeah. That's literally an "upgrade". From the PS2 and the PS3 and Xbox to Xbox 360? That's more like a jump to a totally different more powerful architecture. The original PS2 ran whatever it ran, and the PS3 has those cell processors with immensely more powerful video capabilities. The original Xbox was basically just a P3/733 Coppermine with 64MB of RAM and a custom Geforce 3 in a big, ugly black box. The 360 runs off of of a multicore Xenon processor with ATI graphics.
Saying the Wii U is an upgrade from the orignal Wii is like saying my dual core AMD processor is the same as my old 500mhz AMD K6 processor ... just a little faster. lol