It's because it's a part of our basic law; it's a part of the rules of the game.
That said, it would have been something that Parliament could easily have done before 1982 and there wouldn't have been any question about it. Whether it was a constitutional change or not, who cared? Parliament could do it.
Now it really matters whether it's a constitutional change, and only the Supreme Court can give that answer. My guess is that with their evolving view of constitutional architecture, as they call it, they would probably think it is part....