It is no secret that Nintendo made a big mistake when they decided to release the 3DS version of Super Smash Bros. 4 before the Wii U version. Nevermind the fact that each player needs a 3DS system to play multiplayer, or the fact that this version has much worse controls, vastly inferior graphics and resolution, a much smaller soundtrack to choose from and may serve as an entrance point for a lot of players who had no intentions of ever buying the Wii U version until they had tried out this clearly sub-optimal version of the game. Another point that you would be wise to ignore is that releasing the Wii U version first would severely limit the sales of the 3DS version and the already underperforming 3DS system itself (which, quite frankly, deserves to have a bad year with no system sellers other than whatever that Tamodashi Life bullshit was), as well as the fact that releasing the Wii U version later may give them more time to develop a more balanced and polished product.
Because it was a mistake, I'm telling you!