I wasn't refering to extremely obscure characters; even just some secondary characters such as Tingle in Zelda, or characters from franchises that aren't Zelda or Mario (like someone mentioned, Rhythm Heaven for example).
Captain Falcon was an obscure character back in N64 (even though F-Zero was popular enough, the character wasn't that famous), and Ness was obscure as well; Jigglypuff was not much important in the Pokèmon games, though it's in Smash 64 regardless; 3 out of 12 is a lot.
Melee added the Fire Emblem characters, Game & Watch, Sheik, Mewtwo (popular among fans but still a secondary character), Ice Climbers Pichu and Dr Mario (even if those last two were clones).
Brawl added all the main cast from Kirby (MK and Dedede), Wario, Lucas, Pit (retro character at the time), Wolf, Olimar, Diddy Kong, and Zero Suit Samus.
Smash 4 added some new franchises and unexpected characters, including Duck Hunt, and Wii Fit, that while it was very popular, nobody would have thought that the trainer of that game could have become a fighter; and the Koopalings, and Bowser Jr, and even Rosalina to an extent, who aside of party games, was pretty much just an NPC in one game. Add Shulk to the list if you want, depending on how you consider Xenoblade to Nintendo. Does Palutena count too? Am I forgetting someone? (dark pit is a low effort clone)
Now, most of those characters would have little to no chance if they were not introduced already, because the focus is on third parties.
Not only they didn't add Arms characters (their new franchise), they still didn't add any Rhythm Heaven character, they didn't add any character from the Zelda franchise despite being still full of very requested characters, they didn't add any Kirby characters, they didn't even add Dixie Kong, who's a pretty major character in her franchise. And more, those are just examples; Where is Krystal, where is a Wario Ware character? Where is Chibi Robo? etc.
They added K Rool only because the fans made a big noise, and Piranha plant, that's the only exception. Other than that, only very major characters or promotional/marketing stuff (aside of the echos who are Dark Pit level of low effort); side character/obscure or retro characters "slots" are now taken by third parties instead.
It's not super obscure characters that I'm talking about... it's side characters of popular franchises, or characters from minor franchises but still popular enough.