Perhaps we as as species generally began to gravitate more towards warmer places. Although I am not sure that makes sense, considering so many semi-ancient civilizations existed in places like Norway, Sweden, Ireland etc.
Maybe as we figured out how to skin animals to make ourselves warmer we began to lose that hair. But who knows!
Not really sure why we're having such a long discussion about hair... or lack thereof... Are you complaining? Would you rather have more hair? Personally the little that I have is a pain in the petudy for me and if I had it my way, the only hair humans would have is eyebrows and eyelashes. Lol...