Most important arguement: she has never been in a Mario platformer or 3D adventure game and has always served as the 'princess number 2' in Party and Sport games to applease the female fans of the series who cannot pick Peach because their friend, cousin, sister in law, grand aunt or family dog chose to play as Peach before. She has her fans gained from over the years, sure.
I must point out here that I'm a female fan of the series, and I strictly play as Bowser in the Mario ensemble games (and also formerly Diddy Kong when he was in Mario Kart). We're not some sort of alien race that only picks female characters and which needs appeasing by means such as "This character is here because someone else already chose the pink one", you know.
Additionally, Daisy's debut appearance was in Super Mario Land - the fourth platformer in the Super Mario series (released after Super Mario Bros., Super Mario Bros. 2 J, and Super Mario Bros. 2 NA/EU, and before Super Mario Bros. 3). She originates from the platformer lineage.
Regardless, I don't see how either of these points should exclude Daisy from Smash - they just boil down to "She has appeared in XYZ type of games instead of ABC type of games, therefore she cannot appear". In spite of Daisy's origins in the mainline series, it is true that she's most well-known for the spin-off games these days - but Smash itself is a spin-off game, and actually a smaller one than those that Daisy normally appears in, to boot. It's not unreasonable at all for players who like a character to hope to see a character who's frequently available in most Nintendo-made spin-off titles, in another Nintendo-made spin-off title, as that character, and not as a re-skin for a different character.
But like Waluigi, she doesn't really have any moveset potential that isn't completely made up out of random apperances from rather forgetable games. Hence why I say Peach alt is as good as it gets, with her own voice clips and flowerly effects. I even think that's quite generous to be honest. She's not like Toad, who can just use stuff like the Ice Flowers or Propellor Cap because he was playable in the NSMB series (amongst things).
Daisy has plenty of moveset potential, though. You don't have to make a Frankenmoveset out of only their canonical appearances in order to represent a character in Smash - most characters in Smash don't adhere to their home-games especially strictly. I don't remember Donkey Kong flying around by spinning his body like a helicopter in any of the Donkey Kong games, for example.
Most characters have a combination of canonical moves, and moves that represent who they are, rather than strictly using things pulled wholesale from other games and cobbling them into a moveset that might not even be cohesive for the character.
Here are some examples of moves, which, to make things simpler, I'm going to approach strictly from the perspective of a Luigified semi-clone, here, since we're talking about alts;
Daisy, being tomboyish and more aggressive than the rather more reserved Peach wouldn't slap an opponent in the face - honestly, she'd be more likely to punch you in the nose. A solid punch in the nose would have a very different effect on the gameplay than a slap to the cheeks! It would do more damage and knockback, by the very nature of the sort of attack it is. This isn't from a game that Daisy's been in, just as Peach's face-slap isn't - it just shows a bit of who she is.
Peach can shove you with her butt - but why would tomboyish, showboating Daisy do that? She's more likely to pull out some sort of low-flying ball-sports type kick or somewhat show-offy karate-like kick, which, again, would have different consequences to someone attacking using their backside, which would directly affect how the move is used against opponents.
Daisy wouldn't throw vegetables - she wasn't in Super Mario Bros. 2, or any game with this mechanic at its core. A better thing for her to throw would be the Superball, which was Super Mario Land's replacement for the Fire Flower. But these wouldn't behave like vegetables, because the source-game prescribes a specific means of movement for them - they bounce along the ground, and will ricochet off of objects at right-angles. To expand on the Superball's movement (and its restrictions, such as only one being out at once, and the amount of time that it's on-screen for), and to make it more flexible for the purposes of Smash, you could make some Gordo-like tweaks to it that could change the arcs of the bounces depending on which direction, if any, you input whilst throwing one. This, again, leads to a very different functionality and very different options to Peach's vegetables, which are simply for beaning people with. Moreover, Superballs are non-solid, and dissipate upon contact with an enemy - this would mean that they can't be picked up and re-thrown in Smash, as Peach's vegetables can.
Daisy has no association with Toads, which nullifies her using Peach's counter move. Whilst I've often seen it suggested that she could pull out an enemy from Super Mario Land for a re-skinned equivalent, I've never really understood why she
would: Daisy likes to get her own hands dirty - she wouldn't hide behind someone else in a fight! In my opinion, this opens up a slot for a new move that fits her better, and which isn't yet another counter; Since around a fifth of the roster already have this same move, I don't feel that we need another counter in play.
Further, Daisy hasn't ever been shown to use a parasol or have an affinity for them, to the best of my knowledge. This makes Peach's recovery move irrelevant to her. Now, Daisy originated from a Super Mario platformer that introduced two things that were new to the series - vehicles, and an end-of-round bonus-game that involved climbing ladders to reach bonus items. Daisy could throw up a ladder as a tether recovery, or use the Sky Pop plane for a vertical recovery - either one of these would fit her and work nicely, differentiating her from Peach even further.
Additionally,
Luminario
suggested something visual to represent Daisy's personality, recently, which would alter her dash-attack into Daisy tripping over a frayed edge of her dress - something likely to happen to a tomboyish girl who, by formal requirement, must dress "appropriately" according to her royal position. I commented then, and reiterate now, that this could lead to a move that is somewhat similar to Dedede's klutzy dash-attack, which involves him tripping and falling flat on his face, and has quite a lot of launching and (later in a match) KO power. Again, this represents Daisy fittingly, and is not similar to anything that Peach does, either in Smash or in their home-series.
These are just six quick things off of the top of my head, from a person who doesn't in the slightest bit enjoy coming up with movesets - these sorts of things are simply that obvious to a fan who knows and likes this character.
And even with only these six changes from the "base", this brings Daisy quite some distance, visually and functionally, away from Peach. Considering that Dark Pit and Lucina were separated from Pit and Marth for far less moveset differences, I really don't see why a Daisy who is fittingly-represented should have to be relegated to a Peach re-skin that doesn't leverage who she is.
Rosalina always had way much more to work with, so I do not see why you people bring her up, or King K.Rool as a Bowser alt. It are completely different situations which are not comparable.
For future reference, my name is Tortilla Noggin, not "you people".
Anyway...
Perhaps it's just me, but because Rosalina had mostly only appeared in playable roles in the same spin-off titles as Daisy, I honestly didn't see what possibilities she had going for her until she was already in Smash. I never saw anyone else envisaging a loosely Ice Climbers-inspired moveset for her, either, though that was before I was here, so I could easily have missed this.
And, I bring these points up because Daisy is a character with her own personality and past representations, and is not Peach in a different dress, and I have seen the arguments used against Daisy today, used against other characters I support - such as King K. Rool, hence listing him as an example - countless times in the past.