The thing with Splatoon is that it was a surprisingly huge hit IP made with I assume a small team. It's a small game. A fantastic game, but a small one. It benefited a lot from its post launch content and was satisfying enough, but structurally and conceptually is neat, tidy, and focused in the way any new IP testing the waters is.
It rapidly transformed into a cult hit, has a distinct identity that's still fresh (pun intended) in the gamer mindshare, and has plenty of wiggle room to grow in a sequel.
Mario Kart and Smash are essential to the Nintendo catalogue, but they're intensely established and don't need an immediate, reactive release. They're the kind of games that can get away with early gen ports before main releases come much later. Splatoon, as a new IP that's still buzzing, benefits most from a direct successor coming sooner rather than later.
This is especially true as a new platform launches and the Wii U market transitions over to the new hardware. Smart marketing would emphasise getting on the Splatoon bandwagon as soon as possible with a big new game on the new hardware instead of letting the fanbase linger for too long on a platform left for dead. Splatoon as a franchise benefits far more from an early major release than Mario Kart and Smash, both of which could launch 2 - 3 years from now and still hit as hard as ever.
I want someone to sell me on the fact that it's basically
confirmed there's going to be new fighters and stages in the smash port. For some reason I feel they want to not really change much from a local development standpoint but also I'd be easier to market for Nintendo if there were like an Inkling in the game. I'm excited as fuck.
Nothing is confirmed, not even the port. Everything is a rumour at this point, even if it's a strong one. There is no strong rumour that the port however will feature substantial quantities of new content.