Confusion more than anything. I can`t think of a single reason she would be added. I get that some fans want her, but I just don`t get why. Doesn't she come from the most maligned Star Fox game?
Yes, but Krystal herself seems to be a well liked character, and she's had other appearances to the point where she'd be a major character if they stopped hitting the reset button on the story.
Why do we need a 4th Star Fox character anyways?
We don't, but that's not really a good justification for not adding any given character. Like, we don't need another Super Mario character to properly represent the series, or any new The Legend of Zelda character, or another Kirby character, but that doesn't mean that they would outright be a bad edition.
Isn't the Star Fox series pretty much stagnant at this point?
I wouldn't say stagnant so much as floundering. Kinda like the Sonic series, except less popular and less wildly different between entries. Still, it's not really relevant to a Smash inclusion.
Krystal hasn't been in a Star Fox game in almost 15 years now.
...but here's the real kicker: Star Fox Zero is the only all new Star Fox game we've gotten period since the (almost) 15 years since Command.
Which is exactly why she hasn't appeared in 15 years; they hit the reset button again and now she doesn't exist yet.
Poor Star Fox. You try so hard, and get so far, but in the end, it doesn't even matter.