Hat wearing among men went from near universal to rare within roughly a generation. Several independent changes removed the reasons to wear one at almost the same time.
Cars removed the outdoor interval
A hat protects during time spent outdoors between buildings. Walking and public transport produce long stretches of that; a car produces almost none.
Car interiors also made hats awkward. Low roofs and headrests mean a brimmed hat has to be removed and put somewhere for the journey.
As commuting shifted toward driving, the daily period a hat was useful shrank to a few minutes, and the inconvenience began to outweigh it.
Buildings became warm
Widespread central heating removed the need to stay insulated indoors and made hats something to be taken off and carried rather than worn.
Cloakrooms, hat stands and hat check counters existed because everyone arrived wearing one, and they disappeared as the practice faded.
Once there is nowhere to put a hat on arrival, wearing one imposes a small cost at every destination, repeated all day.
Hairstyles changed what a hat did
Short, oiled hairstyles that sat neatly under a hat gave way to longer, styled hair that a hat visibly flattens.
That converted the hat from something neutral into something that damaged an appearance the wearer had spent time on.
Styling products increased the effect, since a hat both disturbs the shape and transfers residue into the lining.
Formality stopped requiring it
Hats carried an elaborate etiquette of raising and removing, which functioned only while everyone understood the rules.
As dress codes loosened, the obligation weakened, and once wearing one became optional it also became a statement rather than a default.
Statements attract attention, which is exactly what most people avoid in everyday dress, so adoption fell further with each year of decline.
Where hats survived
Function kept them alive wherever it remained. Sun exposure, cold climates, outdoor work and sport all still produce daily hat wearing.
The baseball cap took over the category almost entirely, being soft, cheap, crushable, adjustable and free of the etiquette that governed the brimmed hat.
It also solved the storage problem that finished the felt hat off, since a cap folds into a pocket and survives being sat on, which no structured brim will do.
Which suggests the decline was less about hats as objects than about a specific formal object failing to fit a changed daily routine.