Due to them being in the dark most if not all the time. Along with them being with the often neglectful, horrible wraith kings. Their skin is very sensitive to light. And they the wraith kings ride them because they were raised and made to do so. Thus the lack of their draconic abilities like the breathing of a element(fire, lightning, etc) along with incapacity of speech, and the lack of having a hoard. This also means due to the darkness they have heightened senses such as smell and sight. Often smelling out rotting corpses to get their form of nutrition. Also with their sensitive skin, the Wraith kings will need to make extra padding under it to help them fly and bite comfortably.