Games on the early hardware were not graphically up to standard to show at E3.
Developers were working with underclocked Wii U development kits prior to E3. The hardware that they were using to develop until recently was underpowered compared to the expected final Wii U specifications.
A report from a Wii U white paper produced by Hit Detection, a consulting company founded by former technology journalist N'Gai Croal, states, "Developers have underclocked development kits, and worked hard to deliver titles running on that hardware to demonstrate live at E3." There could have been real demos of third party software at the show, but the graphics weren't much better than current generation consoles (PlayStation 3, Xbox 360), so Nintendo opted to show footage from other platforms at the press conference.
This information could resolve conflicting reports about the Wii U being about as powerful as current generation consoles, and also 50% more powerful than the PS3.
