Shiggy gives a different reason for no online in NSMBWii

[Mr Khan]

So now he's saying what he probably should have said from the beginning, that they *could* have done it, but their priority was crafting a local multiplayer experience

 

ONM: Was online multiplayer considered for the game at all? Was it just too difficult to get it to work without any lag?

SM: Well, it's not for purely technological issues that it wasn't included. After all, as you know, we already have online play in Smash Bros., and technologically speaking, I don't think it's creating a high amount of lag. It's just a matter of our priorities when it comes to the creation of a game. We just prioritised on making a multiplayer mode where four players can sit together and play while seeing each other's faces.

We thought that this was more important as far as this New Super Mario Bros. was concerned, but we don't know about the future. Because network environments are getting better and better every day, it may be possible in a future edition to incorporate that type of multiplayer online mode for a real-time action game. But I am just talking about the technological aspect here. What's more important, of course, is if it's going to add to the fun aspect of the game.

This explanation seems like the most appropriate, though that statement about Brawl not generating a lot of lag has to be a poor translation, or he has really never tried it...

Full interview here: http://www.officialnintendomagazine.co.uk/article.php?id=14117