http://www.at7addak.com/article/miyamoto-explains-why-theres-no-online-mode-for-super-mario-3d-world
When New Super Mario Bro U launched with Wii U I never really considered it a next gen Mario game. Super Mario 3D world on the other hand offered 3D lush environments in HD, a plethora of new powerups—a new Mario game built for a new console. But one feature it does seem to be lacking is online play. Sadly the game will not include the feature despite the fact the Wii U is more than capable to handle it. So what gives? Shigeru Miyamoto, creator of Mario, told Kotaku:
""While online play is certainly technically possible for us, it simply wasn’t the focus for us this time around," Miyamoto said. "What we really wanted to do was to create something that people could experience fully while playing comfortably with others who were nearby them, and this is something we decided would be best for Pikmin and the best case for Mario here as well.P
"Now that is to say the answer to this question might change in ten years time, if there’s a future game where for example we don’t think it’s important to be able to see the face of the person you’re playing with, then we might be able to focus more on some online function there. But for right now our focus is really on a comfortable play experience with people in the same room.""
While at first I'm inclined to disagree I do at the same time get his point. The Wii U was designed to attract members of the house hold to participate on the fun. Obviously if someone is playing online there wouldn't be much reason to invite the ones around you. Smart move, Nintendo.