Mostly because for an unconscious A&E case you have a really limited amount of time to find out "what would this person actually want"; so its good to have a rigid rule that you do your very best to save them unless they have a DNR tattoo or other really very obvious indications they don't want help. So there's a much bigger time pressure.

Also because I think there really is a difference between someone standing there shouting "NO" at you; and someone who can't communicate at all. I don't believe that psychotic patients are utterly incapable of reasonable communication.