One of the things which fascinate me is the wearing of hats in the US.
If you look at pictures showing massed men in the 1920's/30's, as in unemployment lines or baseball stadiums, you see all the men wearing hats. There also seems to be a lot of uniformity in dress, like business suits, but the hats are the easiest to see.
Recently I noticed a picture of Abraham Lincoln addressing a crowd, I think the 2nd Inaugural, and noticed his audience was also wearing hats. The picture wasn't as clear as more modern ones, but it looks as if there's a bit less uniformity in the types of hats being worn. In another photograph his audience in front is hat wearing, the big shots behind him are hat carrying, mostly top hats.
When you google "when did American men stop wearing hats" the first result is an Esquire article saying hat wearing started to decline in the later 1920's. Why--perhaps because more people were in cars so they were less needed and some were more awkward to wear.
This NPR page has good comparison pictures and blames Ike but also cars.
Neither of the pieces comment on the change which seems apparent to me--fewer hats correlates with greater variety in menswear.