I read somewhere they wore shorts to allow them to avoid getting their shirts and pants snagged in the bush.
They got the idea from natives many years ago who could move silently through the bush.
I've seen many pictures of English wearing shorts in WWII also.
A cousin of mine enlisted in the army right out of high school, the brown boot army, 1957, 1958??? He gave me a bunch of his old uniforms and in the pile were a bunch of Khaki shorts he was issued.
Vietnam guys: What about this? Any idea why the US didn't issue you guys shorts too?