"Because they have nothing to gain by answering that question with a "no." Again, keeping Shantae in the discussion about Smash is in their best interest because mindshare is extremely important in the entertainment industry. It can make the difference between an empty seat in a theater and a seat with an angry viewer who already spent money on the ticket. The only thing worse than bad press is no press at all. This very video is literal proof that Wayforward benefits from not answering the question. 16,400+ views means 16,400+ individual moments where a potential customer might have been compelled to maybe look at a trailer of the game out of curiosity.
You may think that's insignificant, but in the world of marketing, that cannot be overlooked. And this is just one video. Companies pay millions of dollars to run commercials that don't even explicitly advertise a product, but instead merely function to create an association between their brand and a positive emotion—to keep their brand in the back of the viewers' minds in the off chance that they might choose to give that business their patronage. If Wayforward already dispelled the rumors by making a public statement, then videos like this simply would not be created, and the conversation would move to other, more relevant characters for whom the question has not been answered. So yes, the simple act of answering the question is actually an act of foregoing all of that organic marketing—a.k.a. "going out of their way." If you think that this is a "stretch" then I recommend that you avoid a career in marketing. =P
Sure, a handful of companies might just go ahead and make a public statement that their character is not in smash. I would hazard a guess that in those instances, the characters in question probably didn't command a significant presence in the discussion in the first place, and therefore didn't have much to lose. Unlike Shantae, who consistently places high in fan ballots. Maybe I'm wrong, so feel free to bring up a specific instance that goes against this particular line of thought.