I can tell ya why - don't take a rocket scientist either :-)
Men 'LIKE' the pin-up calendars - they get their jollies looking at naked or subservient females, so these calendars will NEVER go away. And indeed, if we look at the pro-rated portion of male bashing or victimizing calendars versus the portion of female victimizing calendars - we would see that females are the victims of this at an extremely higher rate than males. And sadly, nobody will say a dam word about female victims - good ol boy's club at work there folks.
See this link (click here) for more info on female degrading calendars. These are more than just 'naked women' though. These are pure abuse in its finest form. WARNING WARNING WARNING WARNING!!!! Extreme photos - not meant for work environment. Not meant for viewing around children either.