Why the heck can you not? Who knows. Thankfully, Nintendo knows about this issue and that people desire it, and is working on an update that will allow Wii U owners to play downloadable titles, including Virtual Console games (once the Virtual Console debuts on the WiiU, that is).
To quote, Nintendo told Kotaku:
“Nintendo is moving ahead with development of Virtual Console for Wii U. We plan to make it so you can play Virtual Console games on the Wii U GamePad alone”.
Reggie, Nintendo of America’s Vice President, has also publicly stated that he would “Love for that to happen” regarding the ability to play Virtual Console games on the WiiU GamePad.
These are definitely positive developments, but I wish Nintendo would get their act together and have stuff like this figured out beforehand. Not later on. Nintendo fans who love the Virtual Console and have downloaded tons of games for it on Wii, DSi and 3DS, should be rewarded for their diligence and fan-service, not “punished”. But oh well, as long as they fix these problems, then all is well. But they need to continue to learn from their mistakes.