Many fans believe that Waluigi was snubbed from the Smash Ultimate roster because Nintendo simply doesn't care about the character. Waluigi has never been a major character in any of Nintendo's mainline Mario games and has only appeared as a supporting character in spin-off titles. It's possible that Nintendo simply doesn't feel that Waluigi is popular enough to warrant inclusion in Smash Ultimate.
Another theory is that Waluigi was snubbed because he's too similar to existing characters in the roster. Waluigi shares many moves and abilities with Luigi, Wario, and even Daisy. It's possible that Nintendo felt that Waluigi wasn't unique enough to warrant inclusion in Smash Ultimate.
Whatever the reason, it's clear that Nintendo doesn't have much love for Waluigi. The character has been largely ignored in recent years and it doesn't seem like that is going to change anytime soon.
Another theory is that Waluigi was snubbed because he's too similar to existing characters in the roster. Waluigi shares many moves and abilities with Luigi, Wario, and even Daisy. It's possible that Nintendo felt that Waluigi wasn't unique enough to warrant inclusion in Smash Ultimate.
Whatever the reason, it's clear that Nintendo doesn't have much love for Waluigi. The character has been largely ignored in recent years and it doesn't seem like that is going to change anytime soon.