By my estimation, it's that the fur faux-robes are too obviously built-in and thick to ignore like Gardevoir's sheer skin-dress that has a convenient gap or Braxien's "skirt", and it's the near totality of how you tell Delphox apart from Braxien so you can't just "strip" it, you have to work around having a built-in made-of-fur full-length robe or go so off-model it's liable to be mistaken for Braxien anyways.
Additionally, Braxien falls much better into standard Poke-fapper tendencies by looking like a bratty anime girl, while Delphox's design is a grown-ass androgynous mage who's girliness is informed mainly by previous stages. Waifu-mons are generally *extremely blatantly* feminine, like Braxian's skirt, while Delphox successfully looks to have a gender-neutral "robe".
>TL;DR, Braxienfags can't wank past fluffy robes and want anime girls
