I believe the real answer is that having restrictions just makes things more simple to work with. When you have to adapt something to each race, that's more work you gotta put in to make it look good (although those Hrothgar hairstyles...).

Some might argue that it gives races "uniqueness", and it does to the extent of "X race is unable to use Y race's hairstyle."

This seems kind of silly to me, however, for I've yet to find a reason why a certain race might not be able to style their hair in a certain way while another one could.