There's a back to basics vibe to this so far, so launching with a new Mario game would fit into that. It sets out the stall for Nintendo Family Fun early and clearly - Nintendo consoles are the Mario machines after all.
Zelda looks awesome, but it isn't the only adventure-y game out there by a long way - whereas Nintendo pretty much have the platform game scene sewn up. From a branding perspective, I can see that playing to strengths early on is a good idea, then expanding.
Smash 4 is a personal favourite (Kirby 4 lief), although it is a thing in esports, so a port would make sense at some point since Nintendo seem to have an eye on that. It isn't everyone's cup of tea, though, true.