Because the idea of Marvel games is that they involve Marvel characters, duh, as well as the idea of Smash is that it involves videogame characters. Once you start allowing other stuff in, the game's core identity changes.
The idea of Mario Kart games is that they involve Mario characters, duh. Did Link and the Inklings change that? It's still called MARIO Kart, right? You still see MARIO characters front and center on the cover. The "core identity" didn't change a bit.
DId Link, Heihachi, Spawn, the female Spartan, Yoda, Darth Vader, Starkiller, Lloyd Irving, Ezio, or Devil Jin change the identity of Soul Calibur? Three of those characters are from comics and film.
Did the xenomorph, the predator, Jason Vorhees, Freddy Kruger, Leatherface or Kratos change the identity of Mortal Kombat? Not a single one of those guest characters are from fighting games... and outside of Kratos, games in general.
Lol at you guys really thinking one character on a roster of 70+ is going to suddenly make the game go from "Super Smash Brothers" to "Goku and the Pips."