There have been studies that prove men are attracted to long hair because it signifies good health and fertility. Subconsciously men are hardwired to notice tiny details in women such as fullness of the cheeks, color of the skin and other more sexual details that indicate a good candidate for passing on genes with a good partner. On a purely animalistic (pre health care caveman days) level a woman that is healthy enough to grow nice long hair is probably not sickly or vitamin deficient. It is a biological advantage for a male to pass on his genes to a female that is healthy enough to carry his offspring to term and beyond.
Sex, desire and fertility documentary on Discovery Health channel.
Evolutionary Psychology: The Ultimate Origins of Human Behavior by Jack A. Palmer and Linda K. Palmer