Fire Emblem isn't a new franchise. And Returns and Tropical Freeze just came out. There's no excuse. NONE.
Here's one, as someone who loves the Donkey Kong series in general:
The franchise tends to be focused squarely on one character, Donkey Kong, with all other characters being very peripheral. This includes characters like Diddy (in for Brawl and Smash 4) who are essentially backup dancers to what is the Donkey Kong show. People would argue for Dixie, but she is essentially Diddy's female alt, and despite being playable, was never really fleshed out as her own character.
From there comes the question of K Rool, who personally I'd love in the game. He doesn't scream playable character in Smash though. I'm sure it could be done, but he, to many it seems, is nothing more than Generic Villain 'a la' Donkey Kong. Here is the concern, the series isn't that deep. It's got great games, but there's noone who really jumps out.
Compare this to other series, such as Mario, where you could make strong arguments for the individuality of Mario, Luigi, Peach, Bowser, Bowser Jr, Geno, Rosalina, Toad and so forth before drawing a long bow, you then have the separate series of both Yoshi and Wario, very popular in their own right, with only one character each. The Zelda series has a strong storyline, and characters like Link, Zelda, Shiek (seperate for gameplay reasons it seems), Ganondorf, Toon Link, Zant (denied), Ghirahim (assist trophy) and so forth are actually struggling to boost that figure. Then there's Metroid with... Samus, and Zamus now (again, for gameplay reasons), but ultimately it's one character represented should Ridley not make it.
The Mario series, as the series Nintendo is known for, and Pokemon, a series that conquers all in it's path, will be given massive representation. Beyond those the real comparisons are Zelda, Kirby and Metroid. Even the all mighty Mario series is so fractured in terms of spin offs (they've only really got Mario, Luigi, Peach, Rosalina and Bowser [5], Yoshi and Wario [assuming he's in] are very much their own series these days). Zelda has mustered Link, Zelda, Shiek, Ganondorf [if he's in] and Toon Link, with all 5 being pretty much certain includes both within their own right and Smash terms. Kirby have, well Kirby, along side King Dedede (many were disappointed he failed to make Melee) and Meta Knight, which considering it's Sakurai's baby and it's place within Nintendo seems fair, while Metroid is stuck on Samus + her gimmick turn character. Donkey Kong is an important series for Nintendo, but who really is that important beyond Donkey and Diddy Kong? I like K Rool, I'd love him to be in, but I just can't see it. Even Ganondorf seems very on edge in the Smash series, and he has as far bigger footprint than K Rool in the gaming World.
Could you imagine the disappointment if K Rool ended up a Bowser Clone...