MK8 did not get a demo. Dream Team and Awakening getting a demo was a treat, but for other huge Nintendo titles like MK8, or Pikmin 3, or Wind Waker HD or any other Zelda game, we didn't get a demo. Us getting a Lego Game or MGS demo doesn't raise the chances of getting a Smash demo, those aren't Nintendo games.
If they made a demo with 4 or so characters some people would just be happy with that and not buy the game, although that wouldn't really hurt sales since I think that would be a very small group of people, I digress. They don't need a demo to sell this game.
I'd love a demo, but don't get your hopes up.