Cult Appeasing Roster.
Casual/party/family appeasing roster.
Explanations at bottom.
Expecting flame.
Snake stays: We don't need any more 3rd party, even if Namco is developing. Snake staying helps keep some "more serious" gamers interested in the game.
Waluigi & Baby Bowser: Casual Party-Players love both Waluigi and Bowser Jr. To them, they think "Why are they both in Mario Kart, but not Smash Bros?" I'm sure there are some kids out there who go through random Waluigi phases, and I know that Bowser Jr. is a hardcore casual favorite, so both should be in. Toad is left out, as casuals might say: Toad can't fight!|
Mii: Every little kid wants to be a part of their favorite game, and little kids play Smash. It means little to us more serious gamers, but Sakurai has explicitly said he isn't aiming to please us, even going as far as to call us "maniacs."
Tetra: Bolder guess & personal preference. I think she's the most likely Zelda Newcomer, and I think a Zelda Newcomer is more likely than, say, Shulk.
No Chrom: Conflicting niche with Marth and Ike. Ike fights for his friends. Quotes = casual love.
Anna: FE warrants a third rep, and Anna fills a niche that finishes off the "FE sword users." She's small/light/fast/flexible/etc.
Krystal: For the casual girls. They exist, there are many of them, and many of them love Krystal. Krystal not being playable was something many of htem hated.
Isaac>Shulk: Popularity, potential, uniqueness, and semi-cult appeasement.
No Ness: Super bold, but I think one will get cut. Ness has veteran status, while Lucas is more recent/etc. Most casuals I've talked to think Lucas is cooler.
No R.O.B.: R.O.B. sucksssss. Nobody likes R.O.B.
No Wolf: I think the Space Animal fans can take the hit. Krystal would probably be a better character in if we're trying to increase revenue and please customers. Star Fox shouldn't even have 3 reps at this point, but I feel it will retain all 3.
No Ridley: He's a boss. Casuals know him as a Brawl Boss.
Appeasing Ridley cult: "Boss Mode" has an option where the user with the Wii U Gamepad can play as the bosses, so you can play as Ridley.
Roster count is 46 + 4 transformation characters, so 50. If Mii is 3 characters for 3 sizes, then "52" characters.