Well, Golden Sun is a niche 2nd party franchise, people were lucky enough to get somebody from Xenoblade, and that's first party. Ridley has the excuse of being too big or being a boss, and even the Metroid series itself continually under-performs despite a crazy loud fanbase online... But there's no excuse to leave out Dixie, no excuse. DKCR on Wii alone sold more than any individual Zelda game on Wii, yet we see more Zelda content. The DK series is first party, is prioritized by Nintendo and is given to the best developers they have (and has been, aside from PAON for the spin-offs), and it has unique characters, plenty of them, and some important enough to be included in Smash, I'd say.
Yes, Star Fox, Mother, and Wario fans should feel disappointed too, their franchises took a step backwards... But those franchises were never heavy hitters like DK, and they certainly didn't have a massive and successful revival during the previous generation (the generation this Smash Bros. game is reflecting the most). For the DK series to get practically nothing out of that renaissance is a HUGE disservice, particularly when franchises like Kid Icarus and Fire Emblem didn't sell nearly as much but get loads more content. It doesn't make sense
DK has both relevancy and legacy, it's not niche, and it has unique characters... What gives, Sakurai? What seriously gives?