Why are 'we' longhaired?

A reader writes:

It may be that very long hair was selected against in the tropics due to either pests or possibly the greater ill effects of dampness during the rainy season. (In the tropics during the rainy season being too covered up tends to lead to rashes and/or poor healing of blisters and other small wounds.)

This explanation makes sense if the short hair of tropical peoples is the puzzle to be explained. Why are they not like us? How did they become different from us? (Evidently, us means longhaired, melanin-challenged folk).

Well, they didn’t become different from us. We became different from them. We are the evolutionary oddity and we are the ones who need explaining.

In scientific jargon, long head hair is the ‘derived’ characteristic. All ancestral humans started off being shorthaired. Then, somehow, for some reason, and in some populations, their head hair began to grow to remarkable lengths. Why? Isn’t that the evolutionary conundrum?