(Let me say to you all that I've been eating my words about Nintendo since I got a 3DS. With that in mind, let's begin!)
I have a few things to back up my claim - probably.
First off, the release date: rumors point to 21 November. That places it - and its hype cycle - ready for the holiday season, all guns ablazing, haters be damned. That means it'll be on the minds of anyone buying a Wii U considering the advertising. Plus, you still have the bundles to consider.
Then, I can point to the launch of Mario Kart 8, and how well it's been doing at this point. We're seeing it home in on a 40% attach rate on Wii U, which implies that the holidays may not have as big an impact on it relative to other games (seeing as a lot of Wii U owners own it by this point). Also, I expect the impact of SSB on sales to mirror what MK8 did, except amplified by the holidays.
Then, I can point to the 3DS version. The demo actually managed to turn some of my friends* off, and considering what's happened with the slide pad in Japan at this point, I can see people skipping the 3DS version entirely and waiting for it to come out on Wii U.
The 3DS version can and will compete with the Wii U version - it happens on multi-plats. But, we're seeing a supposedly inferior version of Smash on 3DS getting released prior to the definitive Smash on Wii U. (Plus, GCN controllers on Wii U.) Nintendo is probably trying to milk $100 (or $400, depending on whether you have a Wii U) from fans of Smash.
(I might actually be willing to bet on this, of all things.)
I WON A BET AGAINST AZUREN! WOOOOOOOOOOAAAAAAAAHHHHHHHHHHHHHHHHHHH
:3