It always amazes me how men in the American past wore hats everywhere they went, like go back to, for instance, 1910 New York City and try looking for a man that’s not wearing a hat. It will be difficult. Example Nowdays, go absolutely anywhere and look for a man with a hat, it will be a challenge.
Why did this transition happen over time and what caused it?
The Hat Thing! Anyone else with more insight on the matter is more than welcome to provide further answers; for the meantime, I commend to your attention this previous post from u/mimicofmodes which deals with the Hat Thing.