I think I read somewhere that the head is very nutritious (And maybe even tasty) for them, and so they eat that first. (Although apparantly that is the same reason so many other predators go for the internal organs, so who knows)
I assume why they don't move onto the rest of the body is that they are stuck on the ground longer if they do so, and are therefore unsafe from dogs and such. I don't know for sure- maybe half of it is that is just what they grew up doing. It's habit. Maybe if they could lift it into a tree they would eat more, like a hawk does on occasion.
I don't know for sure though......I could be wrong.