Honestly just because a very, very vocal minority (yes, minority, even as high as 49% can still be a minority and I doubt half of the fandom felt this way) was dissatisfied with the base roster doesn't mean characters like Wii Fit Trainer or Duck Hunt goes against what Smash Bros is about; if anything they're exactly what it's about. Sakurai has said several times that it's about celebrating Nintendo's rich history. Pitting the on-screen avatar from the Wii Fit series against the dog from that old NES game in the Animal Crossing village using Pokeballs and Super Mushrooms is the very definition of what makes Smash Bros so great. It gets 2 different characters from 2 ends of Nintendo's rich history together in one game.
Does this mean King K. Rool didn't deserve to get in? Of course not, he was the primary antagonist for the DK series for years, it would give the DK series a 3rd rep and add in another villain (2 things we really need). But that doesn't mean characters like Wii Fit Trainer or Duck Hunt didn't deserve to get in. And I wish people would stop acting like Smash 4 is the first game that's done this kind of thing. Game & Watch, R.O.B., the Ice Climbers, Marth, Roy, Ness, Lucas, and Pit were all pretty out-there characters when they were first included in Smash (did anyone other than a small minority on the internet know what Mother 3 was? Who had ever played a Game & Watch game when Melee first came out?). And they're part of what makes Smash so great. Fire Emblem and Kid Icarus wouldn't be relevant today if not for their Smash inclusions. Yeah there hasn't been another Ice Climber game in 30 years and Earthbound is all but dead now, but still. And the Wii Fit series isn't even obscure, there's been several of those games since the Wii first came out, and I bet more people knew about it than people did about Fire Emblem when Melee came out or an obscure NES peripheral when Brawl came out.