The general rule of thumb when it comes to emulation is that the system that's doing the emulation has to be around 10x more powerful than the system that it's emulating. Original Xbox games cannot even be emulated right now, and PS2 is very touch-and-go right now. Right now, the Wii U would be viable for perhaps a Wii U-mode Gamecube emulator, but that's about as far as you can go. PS2 if you really push it. It won't do much more than that though.
Besides, the PS2 runs MIPS, while the Wii U runs PowerPC. The architectures are completely different - therefore requiring even more power to emulate since the instructions for each are different as per the architecture and processor. It's like asking an old iMac to run Windows 98. You simply cannot do that because the hardware does not understand it. You have to write code that can translate it, and those aren't generally too efficient. The Wii U simply is not powerful enough.