Clothing styles have changed significantly over the years and not just for bodybuilders. Most people dress much more casually then they did a few years ago.
Believe it or not, I remember a time women didn't go out in public in pants. It was not uncommon to see women wearing hats when out shopping or going to lunch with the ladies. Jeans on adult men were reserved for farmers, gardeners and other working class blue collar folks while they were on the job. Everyone was "dressed to the nines" at weddings, funerals, dinners out and when going to church.
Bodybuilders wore posing suits which covered a lot more skin, front and back and which nearly came up to their navel. In fact, at State Beach and muscle beach in Santa Monica, CA the police and lifeguards carried rulers and actually measure the narrowest point on the side of swim trunks if they thought they looked to be less then 5". Less than that and you be cited for indecent exposure. Likewise, a two piece suit on a woman was considered daring and immodest.
Yes, I am that old.
I'm old enough to remember. As late as the 1979 if you got on a plane you dressed up. Women wore heels and a dress. Many guys wearing dress pants and shirt at a minimum. Now when you fly it's like you're taking a subway train. I don't know if it's completely true by my friend from Spain says they immediately know who the Americans are. They are the ones wearing crappy jeans, tee shirt and shower shoes/flip flops.
Reeves and Park knew how to dress with style and class.