I think it's because the leather can handle the glaze better than canvas. For years I collected Coach leather bags and I never once had any glazing problems. I sold all of those bags off, but the glazing was always in tact and the bags still looked perfect. LV might use a type of glaze or coating that seems to work for a shorter term on canvas. I don't know if they have always used this particular type of coating or if the formula has changed over the years. It could also be due to other factors such as weather (extreme heat, cold or humidity) and storage/usage of a handbag.
I know there was an issue for a particular batch of Empreinte Speedy bags where the glazing melted on the handles (maybe also due to the reaction of natural oils/body heat with constantly grabbing the top handles). I haven't read about that many complaints on glazing of Empreinte items in general though.
I think you’re right that the leather can hold the glaze better then canvas, by the way a friend of mine had a coach leather bag a while ago and the leather always looked so perfect! It really might be that LV uses a glaze that works on canvas for a short term, I have a feeling that their formula has changed over the years. I hope they re-formulate the glaze so the PM wears and tears better because for the price of this bag you’re supposed to get a bag that will be in good condition after a year but like you said it can also be due to weather and storage of the handbag. I haven’t read any glazing complaints on the empriente either, is it possible that they have a different formula for the glaze on the empriente leather vs the canvas?