Somehow, this can all be blamed on this alleged character named "Waluigi".
The producer of the Paper Mario series has outlined some of the restrictions placed on the series in a new interview, including how existing Mario characters can be used.
Speaking to Video Games Chronicle, Kensuke Tanabe confirmed that the Paper Mario games require new villains because existing Mario universe characters can't be modified, and they also can't be brought into the main Mario games. This is a rule dating back to at least 2011-12, during the development of the 3DS's Paper Mario: Sticker Star.
"Since Paper Mario: Sticker Star, it’s no longer possible to modify Mario characters or to create original characters that touch on the Mario universe. That means that if we aren’t using Mario characters for bosses, we need to create original characters with designs that don’t involve the Mario universe at all, like we’ve done with Olly and the stationery bosses." - Tanabe
The interview also touches on the complexity of Origami King compared to that of prior games.