I'll say this. Smash is built on a cast of characters one could debate all day on the degree "deserving" each character has. I mean, when I first played Smash 64, I had no idea who Samus, Ness, Captain Falcon, Kirby, or Fox was. Yet BECAUSE of Smash, I have played F-Zero, Mother, and Star Fox; and Metroid has become my favorite single video game franchise/universe as a whole, and the Kirby franchise led to some great memories and some childhood friends. None of that would have happened, probably, if it weren't for Smash.
On the same note, Fire Emblem would likely not be as popular in America if it hadn't been for the inclusion of Marth and Roy in Melee. I'd never heard of Sheik or Ganondorf when Melee came out, and then when I saw Mr. Game & Watch I was like "what is this??"
Brawl took less risks in that regard, I feel like. You had ROB who'd previously been re-introduced in Mario Kart DS, Lucas who people just took as "Ness but different" (it seemed like people were less shaken by his introduction being from a familiar franchise, same goes for Ike) and Pit. Pretty much every other character in the game was a pretty well-publicized one even if less popular like Olimar. And... to be honest, I think Brawl's roster was pretty boring. Looking back on it now, it almost feels like given how much Melee expanded the roster from 64, Smash 4's roster is more like what Brawl's should have been. Personally, I like the addition of new characters nobody's heard of as long as they're interesting and their franchise has a strong future. That's why I'm particularly happy Shulk made it in--Xenoblade is a quality game, and its sequel so far is shaping up to be a great successor. Shulk may not be in the game himself (that we know of), but at least Xenoblade is getting a new game and offering new experiences to the player.
As much as I love F-Zero and Captain Falcon, I can sort of see why that franchise hasn't gotten more roster attention. Falcon really is about as iconic as the F-Zero series gets, but doesn't have a lot to his name other than being a racer and bounty hunter. It's sort of difficult to try to put weight on a series that's not very combat based. Captain Falcon's moveset is entirely irrelevant to the series, so my understanding is that he's there more as a staple/icon/fan-favorite/classic to represent a series that's difficult to properly integrate into a fighter/party game.
As to why Wolf and Lucas got cut and Metroid didn't receive another representative, I'll never understand. Huge missed opportunity in my book, but at least we get Mewtwo? Wish we got just one more DLC character after that so the roster selection was even/full.
But anyway, my point is that Smash isn't about how many popular Mario characters can get stuffed into the game. It's about finding balance between properly representing popular series and also supporting diversity and unique franchises that maybe not too many people have heard about but Nintendo sees promise in, a la Xenoblade in this game, Kid Icarus in Brawl, Fire Emblem in Melee. I mean, if there aren't boundaries being broken, then what's the point? If only the most well-known characters get in, then it doesn't really serve as a marketing tool, which Smash partially is.
Also, there's always going to be odd-duck characters like Wii Fit Trainer, Duck Hunt, Game & Watch, Ice Climbers, Jigglypuff, ROB... To not expect them is to not be in the full Smash spirit, in my opinion.
As for me, I think Smash 4 has the most interesting roster so far, compensating for the fact that, for example, Melee wasn't going to be able to have 50 characters. Even if I hate that Lucas/Wolf are gone and some of my favorite franchise went unnoticed, I really enjoy the spread of characters brought to the table in this game. Clones are sorta welcomed since they didn't hinder the development of other characters too much.
Anyway I gotta go, but that's my take.
better than 
this thread is called unpopular opinions for a reason
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