Men in many professions (bankers, lawyers, politicians, etc.) still wear full suits every day, don't they?

Suits are still completely normal in much of the world. Hats ceased to be worn gradually, starting in the 60s. I have no sources (because any answer is really pure speculation) but fashion trends are set by the leading figures of the time, and JFK is often claimed to be the man who made it acceptable to cease wearing hats. It's possible that he kicked off the social trend. I've also seen it suggested that hats ceased to be worn as they couldn't be comfortably worn in cars on a commute, unlike on public transportation which generally has higher roofs, and the large uptake of personal car ownership started in the late 50s and early 60s.

I was watching a documentary on men's fashion. They stated that hats began to fade after WWII as more men began working inside than out in fields, construction, and outdoor work, more people had cars than walk or take a bus when a hat was needed to help keep warm and dry. Also, some men resented being told to wear the in the service. Plus, a large hat is just a pain in the ass.