Three years earlier, President Kennedy went without a hat, causing a loss in hat sales.
Hat sales were already on the decline through the 1950s, driven (pardon the pun) in large part due to car culture.
Look at the roofline of this 1936 Ford Deluxe:
Now, look at the roofline of this 1957 Ford Fairlane 500:
With the '36, a man of average height could get in it with a hat on- with the '57, that's not happening. Sure, he could put it on the front seat, but not if his wife is there. The dashboard? It will probably fall off, and get in the way unless this gent specified the three-speed Cruise-O-Matic. The hat is most likely to get tossed in the back seat.
In my hometown of Toronto, the police force switched their headgear from the Custodian (Bobby) Helmet to forage caps in 1947, largely because the old headgear wouldn't fit in the new cars that were being ordered. There were actually public complaints about the shift, with some people saying that it made the police look less recognizable and too "Americanized"!
As for suiting, scare materials during WWII definitely percipitated a shift towards less formal clothing. For much of the early 20th century, for the upper middle class and up, there were several dress codes for men- and womenswear was even more complicated. The setting of the event was important, as was the time: an event that commenced before 6pm was considered "daytime", and commencing at or after 6pm "evening".A man of society would be expected to have the following:
Formal/Full Formal:
-Daytime: Morning Suit. A long, single-button tailcoat with a curved cutaway forward, in black. Trousers were striped, grey, and held up with suspenders. A white or cream-coloured vest would be worn, along with a long necktie, commonly black/grey striped. Shirting would be white and wing-collared. Shoes were to be highly polished Balmorals. A top hat would also be worn.
-Evening: "White Tie". This was probably the most restrictive order of dress. It required a black or midnight blue double-breasted, peak-lapel tailcoat, worn open-fronted. Lapels were finished in silk or grosgrain. Trousers in matching colour, held up with braces. A white vest, shawl-collared vest, fastened with studs worn regardless of weather, although it could be backless in summer. Shirting was invariably a crisp white cotton shirt with a solid or bib front, fastened with studs rather than buttons, and a fixed or detachable high wing collar, with a self-tied white bowtie. Shoes were highly-polished
never patent leather Balmorals or opera pumps. A black silk top hat and white gloves were worn when outside; in winter, a white silk scarf and Chesterfield overcoat were also worn.
The next step down was Semi-Formal
-Daytime: Stroller. Essentially a shortened version of the morning suit, with a thigh-length, 3-button coat worn in place of the morning coat, and a bowler in place of the top hat.
-Evening: Tuxedo/Dinner Jacket. Similar to what exists today, but with more rules. It required a black or midnight blue jacket, fastened with a single button, with peak or shawl lapels finished in satin or grosgrain. In tropical climates, the jacket could be white or off-white. Polished or patent leather black shoes. Shirting was white with a pleated or bib front and wing collar, with self-tied black bowtie. A black vest or cummerbund was also worn.
Below that was Informal; the business/lounge suit. Even then, there were still rules. Black suits were exclusively for funerals or the help. Pinstripes were for business only; chalk stripes for entertainers and gangsters.
Then came Casual, which permitted sport coats and odd trousers.
At the bottom were workwear, athleticwear and loungewear, task-specific clothing that one would change out of when not engaging in a particular activity.
Even postwar, the shape of the suit evolved. The typical 1940s American suit had high-waisted, pleated and cuffed trousers with wide legs, a long jacket with low gorge and soft shoulders, and typically featured a short, broad necktie. The most exaggerated form of this style of tailoring was the "Zoot Suit". Postwar, most suiting is some expression of the three most common cuts. There is the American cut, which features generously-cut trousers, and a longer jacket with natural shoulders and minimal waist suppression. The "Italian" or "Continental" cut features closer-cut trousers, and a shorter jacket with highly-structured , heavily padded shoulders and a highly suppressed waist. The "English" cut fits between the two, with trousers that are neither tight nor baggy, and a jacket cut closer to the American style but more form-fitting and with more structured shoulders. The American jacket will usually have a single vent, the Continental none or two, and the English usually two.