My guess is that it's because the archaic nature of the style distracts more from their failed state of a body than a more contemporary fashion. Remember that curvy went out of style for a bit until coming back with J-Lo. Twiggy showed up in the sixties and had a lot of influence. Pinup/50s housewife is the most modern they can get while still being an alternative to the thin-waisted Kim Kardashian ideal.