I would assume people prefer to not wear helmets not only because they don't like the way it looks or feel, but they may also believe that it obscures their vision to some degree and while it protects them from accident it increases their chances of being in one as they are less aware when they wear one. Also they may not like having to carry it with them when they get off the bike. It is nice to not have a bunch of crap with you when you get where you are going.