I'ven't read any theory, this is just speculation, but the metaphysics may be something like this:
Jewellery: There are given stones/emeralds that are supposed to have different energies. Muay Thai and Filipino fighters wear them or embed them into their skin. Maybe jewellery may give off different energies undesired for qigong practice?
Eyeglasses: I find it more relaxing to take off the glasses and just relax the eyes into the distance when doing zhanzhuang. This is the natural state and maybe it is thus preferable to the 'unnatural' state of wearing eyeglasses.