The single most significant change in the Attire language over the past 100 years has been a steady up-tick in our comfort level about body display, and overt sexuality in apparel. Of course, women have traditionally been packaged for display of their charms, but now men are doing the same thing. We have watched over the past century as women's skirts have risen to such heights that they nearly vanish entirely, and during the 80s it was possible, through the popularity of spandex, to be able to tell a guy's religion from across a street.
Well, the beginnings lay in the early 20th century, when women started moving into the work force in great numbers, and demanded clothing that would facilitate ease of movement. The first World War encouraged this even more, since so many women took over men's jobs. It was game on from that point.
But it was the 1960s, the Sexual Revolution, the Peacock Revolution, and the Youthquake, that changed everything for the guys finally. Shirts worn open to the waist, pants so tight that their pockets were utterly useless, and the sudden appearance of brilliant color and pattern in menswear allowed men to strut their stuff as they hadn't for hundreds of years.