But cars did also become widely used in those other countries during that time. I don't think that blaming presidents makes sense (and that part was a joke, anyway), but cars as an explanation for the decline of hats seems intriguing. As you noted, in some places in the west, commuting by public transport remained widely used (though I would argue that even many of those people frequently had to use cars), but here I would simply add fashion to the cars and turn it in a two-step process: cars made the use of hats decline, that decline became visible (either through interpersonal relationships or the media) and through that became fashion.

It's an intriguing idea, certainly, but obviously without any empirical validation (that's just how it is, that doesn't mean it's not allowed or fun to speculate about).