Well its because of the little conflict that happened back in Ireland between religious groups. The Protestants were Red and The Catholics were green actually i think its vice versa. now its because to sell inventory on green items so people can celebrate that holiday and so the stores can make money. Frankly they need to bann the hole pitching if you arent wearing green its rather rude! if you are wearing something that is close it should be fine.
Saint Patrick's Day was created by American business men. It was not celebrated in Ireland as anything other than a saints day until the 80's. FYI all saints have an assigned colour Saint Patrick's is a shade of blue! Plus he was on the Isle of Iona before he going to Ireland. Did anyone know he was originally Roman?