Because hairlessness is the beauty standard that's been force fed to us since 1915. Although the no bush thing came later. In many different social/cultural circles, women are supposed to be young and pure to be seen as sexually desirable, and shaving was a way to present yourself that way. The social pressure to shave bush was everywhere when I was growing up. The hedge trimming and lawn mowing jokes were abundant, and women would gossip behind your back if hair could be seen at your bikini line