《礼记·问丧》
回答是： 冠，是最尊贵的头饰，当一个人赤膀露肉时是不能戴冠的，否则就是对冠的裹读，所以特地制作免来代替冠。
曰：冠，至尊也，不居肉袒之体也，故为之免以代之也。
The answer is: Crown is the most noble headdress. One cannot wear the crown while one is bare naked, otherwise it would be disrespecting the crown. Therefore, a wig is made to replace the crown.