If the pumpkins were commercially grown, that may be why. Quite a few people have commented that their animals won't eat pumpkins grown for fall decoration. I wonder if they are laced with insecticides or some such thing to make them unpalatable. When I cook a squash, I usually cook it in the skin. The birds then get all of the guts, as well as the cooked skin. Dog and chickens split the cooking water. Nothing goes to waste.
The fall decoration pumpkins may have lower nutritive value, at least in comparison to pie pumpkins. That can be important when birds already consuming a lot of low nutrient density items.