Probably because he's the one who got his place taken by Sora and a Potted Plant. Also because he's the Nintendo character who gets the least popularity, love, and respect.
Who exactly are you calling a Karen?
I'm not a girl, for one thing, and second, I'm representing a majority. I don't really care if Waluigi gets in Smash. It would be cool, but I'm fine if Nintendo never adds him till the next Smash game.