How does a Nun's habit come to be ociated with the Virgin Mary?

Habit are worn by a specific order so that they can suppress physical beauty. The purpose is to maintain a simple look where everyone looks the same. Today there are many orders that do not wear a habit but do maintain a modest uniformity. The point here is that nun take a vow of chastity, poverty and obedience each order is different and their habit is determined by the order. I hope this helps.