Since you bring up Hollywood, there is another theory that might help explain it. With the rise of movies then TV, you had more people seeing men on screen wit

Since you bring up Hollywood, there is another theory that might help explain it. With the rise of movies then TV, you had more people seeing men on screen without hats. Why? Because it's difficult to properly light a man's face while he is wearing a hat. The lights must be somewhat above the actors, so a hat causes a shadow across his face. If you want to see the man's face, then he can't wear a hat on camera. So, once you have the trend setters removing hats, then people get more used to the idea of seeing a man's head sans chapeau. News anchors still dress nicely in suits, so they would still be somewhat formal, even without the hat. If this were the case, it would probably start to show the decline of hats earlier than the rise of automobiles, in the 1930's or so, so this theory could be tested separately from the rise of the automobile.