Krystal at one point had a strong chance at being playable. Prior to any announcement about Smash 4, she had little competition in the "female" representation department, she was even more popular than she is now, and Sakurai dropped a *hint* The "hint" was more of a nod, Sakurai acknowledging:
1. Her in general (which is good)
2. She can fight. (which is great.
It became clear post E3 and after a few interviews that the roster this time wasnt going to be about filling in existing franchises, at least not nearly as much as it has been in the past. He also noted the lack of.. well... notable characters, so he went for some random / surprise choices. Lastly, he looked over choices in favor of gimmicky movesets. Not just unique, gimmicky.
If Smash 4 would have not taken such a new turn I think she would have been much more likely, but who knows? Maybe she would have been a semi clone or something yucky like that. I still think she has a shot, just... it's clear Sakurai has another vision for how this roster should develop this time.