Well, it's like -
@Xystus - said... it was (or IT IS) a symbol of defiance, the proper skirt length it's around the knees, not more long or short.
Of course in that time
(trying to imagine myself being a woman around the '60 or '80) you just
COULDN'T have a skirt that short or at least less than your knees without attracting or expose yourself from some unpleasant people or even thoughts around you... And at that time the laws about the sexual aggression or even sexual harassment were almost non-existent.
You were (at that time being a girl) trying to oppose against something, NOT EXPOSE against something.