Because this isn't a preoccupation with self image and people with it aren't overeating, avoiding or purging food. Consciously anyway.
I asked my mother and she said although it says eating disorder in it's name, it technically isn't, it's a type of sleep disorder.
I'm writing up a thing about eating disorders and I don't know whether to put it on the list or not!?
I'd count it as both. Clearly, the subject has some sort of a want for food, hence the sleep eating, but it's also a sleep disorder.
So I would imagine it's a hybrid disorder.