So, the 3DS version of SSB4 will come out a LOT earlier than the Wii U one. Considering how desperately they need to sell the Wii U, and do so NOW, why in the world would they do that? The only reason I can think of is to try and get people even slightly interested in the 3DS version, based on the notion that we all want this game very, VERY badly - and THEN get us to buy the game all over again on Wii U this winter. But... that's a gamble if I've ever seen one. I wouldn't be surprised if many people who really want their SSB fix go for the 3DS version, then pass over the Wii U one (which, in some cases may mean the Wii U altogether).
Right, so... nope. Can't think of any logical reason. Nintendo marketing department, you dumb schmucks. Monkeys on typewriters would do your job better quicker than they would write a single word in Shakespeare's works, let alone all of them.