Just saying "JFK did it" isn't an answer; I mean, why did JFK suddenly decide it was acceptable for a President of the United States to stroll around hatless? Something must have created the conditions that led to that being an acceptable decision for a politician looking for re-election; the hat was already on its way out among men of Kennedy's age at that time.

If Barack Obama decided tomorrow to stop wearing pants, would this become a new trend? Or would we be very quickly getting used to the words "President Biden"? The time is not ripe for Presidents to stop wearing pants.

On the other hand, Obama has frequently been photographed with a suit and no tie; as far as I know he's the first President to attempt that look. But it's been a fairly common look among youngish men for well over a decade.