I have read a cause of this superstition, in the ancient times people believed that Wednesday was the day of growth and development, it was also considered auspicious time. The members of royal family and noble men prefered to have a haircut on Wednesdays, therefore there was a rule to prohibit common people to have a haircut in the same day. It was consider inauspicious event to do the same activity in the same day as if you tried to compare your prestige to the royal family.
Another cause I was told is in the old time the hairdressers were brought to the palace to do the haircut for the members of royal family on Wednesdays. It took a very long time to do one haircut because it had to be done with extreme precision, so the hairdressers were not available for common people during Wednesdays.