Why can't you have a rematch with the gym leaders, elite four, and champion in Diamond and Pearl?

Question

just wondering.

Answer 1

You can't rematch Gym Leaders in Diamond and Pearl because that's how GameFreak decided to create the game.
However, you can rematch the Champion and Elite Four.

Source:
Knowledge

Comments

but i already knew that you could rematch them, but what i dont like is that they don't get any new pokemon.