I will never understand why Sakurai put Piranha Plant before not only Waluigi but also Toad, Paper Mario, King Boo or basically any established Mario character.
Add me to the list. I don't expect him but I still like this character and want him.
Because it's not that simple.
PP was chosen as a bonus DLC charactr first and foremost. It already wasn't supposed to be a Hero intentionally, so Toad and Paper Mario were never on the table. King Boo is far harder to make a moveset out of, due to not having as many abilities among the species(a ton of PP's moves come straight from the games). Waluigi also suffers from this, having a fairly small overall movepool at this time. Sports are nice, but they aren't specific to him. Meanwhile a lot of stuff is specific to the various Piranha Plant species, making it way easier to make a unique moveset. It doesn't help that the sports games do have representation via Peach(and now Daisy's) Smash attacks. They seem small, but these are still there for just that alone.
So for Waluigi, just having sports is not enough. He has air swimming otherwise, and would still need a base to be easier to create(PP used Bowser Jr. for the vehicle part). The former clearly cannot be a clone or echo, and his design means they'd have to effectively take some of Wario's janky movement, and maybe Luigi can help provide a skeleton for him. But we don't know if that's good enough. That, and another thing was being iconic and not being a hero. Let's be honest, PP is more known than Waluigi or King Boo by far. It's a very iconic mook since the first game(and he does note that being iconic was part of it).
Frankly, Waluigi was not in a position to be a bonus DLC character like that. PP also has a lot of evidence it was never a base game character, meaning Waluigi was already an AT, so he wouldn't be re-considered by then, as it's all decided while the base game is being finished up(2017 July is when DLC was determined, with PP being the first character anyway). So that at most left King Boo, and it seemed like Sakurai actually wanted more of a mook than an established character at that point. Then again, there's no difference for Smash. A character is nothing more than a single entity in a game when it comes to how simple Smash is with it. So it doesn't matter how established they are. Do they have a name(even for just a species), moveset potential? Yes? Then they're eligible. It's not that odd if you look at the entire picture. It's also why it's a bonus dlc, not a core one, since it's a special situation. The only other one honestly that sounded plausible at the time would've been another mook in a similar situation(like Shy Guy, though that's not as known as the Plant either). Slime would've worked too(though moveset is far harder), but it's more like it was a first party bonus dlc character, or at least second party in the way Mewtwo was. ...Who thanks to the games and even movies is more of a species. There's been established as more than one, so it's not as different from PP either. Albeit, there's two common things; both were bonus DLC meant to make people get the game(s) early, and both are species. Well, a third thing is they're both rather ionic too, yeah. Mewtwo's only core difference is that they actually wanted it in base(the veteran factor is why). PP doesn't have that factor either.
