As a brother of a religious Order who regularly wears his habit, maybe I can give a little insight to this issue. A habit is not simply a distinctive mark of an Order, but rather holds a deep meaning to the Order. Each piece holds a specific meaning which is dear to their hearts and charism. Many Orders withhold certain parts (or the entirety) of their habits until major milestones are made in the spiritual formation of the individual, like simple or solemn vows. If laity simply begin to wear the habit, then it could be taken by the Order as demeaning with regards to the process of formation the religious goes through. Put yourself in their shoes. If they take four, eight, ten years or more of formation in Order to be allowed to fully wear the habit of the Order, then what would it feel like if one of the laity just puts it on one day and starts to wear it.
If you feel called to wear the habit of an Order, you must receive not only permission from the Order but also go through formation in order to prepare yourself for the life which the habit symbolizes and even then, you must be invested with the habit by the Order. The habit is not a symbol but an integral part of the identity of the Order. It is only theirs to dispense.