They're probably thinking announcing DLC before release will do more harm than good to sales. But given:
- Mario Kart has DLC
- Mario Kart didn't have announced DLC until after launch
- Smash, like Mario Kart, is one of Nintendo's flagship series
- Nintendo recently announced their shift towards "non-casual" (read: "high-paying") customers
- (assumption) the higher-ups at Nintendo have more control about what games have DLC than the game directors
I think it's pretty likely, but, like Mario Kart, a long-run thing that we won't see right away.