I'm not asking why "your honour" is used to address judges. I'm asking why "your" is used. When I say "Your car", "your self-respect" or "your peace of mind", I'm talking of something that belongs to someone. Even if "honour", "majesty", "highness" or "grace" is a quality of the person you are referring to, when addressing the person I'd be inclined to use the adjective "Honourable George", "Majestic George", (I don't know what to say other than "High George" :-) ), "Gracious George".
You'd never see anyone saying "Your Doctor George" or "Your Mr. George". It just sounds silly.
So why is "your" used?
Update: From the comments here, and here, it appears to be a metonymy or as @tchrist mentioned: "Because it is too intimate to use the second person to address someone that important so directly. The style of instead addressing that elevated person's "grace" as a form of courteous address allowed for the use of more distancing third person forms". Later, @Lambie mentions: "...addresses the person's station, not the person directly. It's as if there were a cloud around the person of grace, highness or majesty, and that is what is being addressed".