It has to do with the thickness of the material and being able to read it with the densitometer probe. If the material is too thick you will have extraneous light going into the probe, plus the aligment of the sensor will not be the same as if it was a thin material, for which these instruments are designed.

Suppose you turn off all the lights to read the glass or acrylic filters, then at the same time you have light from the densitometer bulb escaping due to thickness.