This is sort of old news, but in an article on Gamerant, Masahiro Sakurai talked about the development of Super Smash Bros. 4 and said that the final roster was finalized two years ago. All of those campaigns and petitions for certain characters to be in the game (including Krystal)? Unfortunately, they couldn't be used. Not because Sakurai doesn't care about fan requests, but because it takes so long to make a game. It takes a while to balance every single character and their interactions with each other. Because of this, the character lineup must be finalized a great deal in advance.
So Sakurai wants to tell you guys "no hard feelings". Maybe next time, or in DLC if they can test it properly.