People who prefer consoles more than handhelds will by the console. Form factor is not the only reason why people buy 3DS over Wii U, price and game selection also are major factors and Wii U is about twice the price with a fraction of the amount of games.
In Nintendo terms there aren't very many people willing to buy their console as is, the portables are overwhemingly more popular and if the portable can now play all the same games ... I dunno.
I think there is a controller gimmick or some kind of novelty at the core of this design too, so perhaps it being something that can be moved easily around the house is part of that. Nintendo may have recoginized that just releasing another portable wasn't enough in light of smartphones/tablets, they need something else that makes it stand out, other than just "well you can also play these games on your TV". That's a decent feature, don't get me wrong, but it's not "OMG! I have to run to the store and buy this now" type thing.