My best guess is, according this comic’s author, the princesses only exist as romance objects for Mario and Luigi. Since Wario and Waluigi are a couple, there is no reason to give them princess sex objects. The princesses apparently are not character in their own right to get mirror versions of.
samus12345@sh.itjust.works 1 week ago
Yes, it’s explicitly stated when they say “You two are supposed to be exactly like Mario and Luigi…”, meaning their whole existence is supposedly anchored to them. It fits for Peach, but less for Daisy, as she was created for Super Mario Land and later was used in the Mario sports games as a counterpart to Peach. The Luigi connection was bolted on later. In that way, she’s a lot like Waluigi, only existing to be a sports partner for another already existing character.