I'm a specialist in a different area, but a lot of times these traditions are "diffused", or adopted/forced upon/passed down from one culture to another. That is often the reason for similiarities in attributes that could otherwise vary extensively throughout different cultures.
For example, sometimes families will branch off and move away, but still keep the same styles as the main family. It may be, as someone said here, for practical reasons, or it may visually associate them with the main tribe or an admired tribe (much the way your last name identifies you as part of a larger family branch).
In Mexico, Aztec women had special hairstyles for women who were married - - kind of like our wedding ring today - - to let other men know that they were off limits. Hairstyles even today can reflect (to some extent) social status, job type (think Armend Forces) or just plain trend (the cuts of the stars, like the old Farrah Fawcett or Dorothy Hammel looks).
Your question is a really great one, and there definitely ARE specialists out there who must have their own ideas as to what the answer is. I hope you have success in finding them!