Here's why:
* Story Progression: The main story in Pokémon LeafGreen is linear. After you defeat a gym leader, you move on to the next one, and the previous gym leader won't be available for re-matches.
* Game Mechanics: LeafGreen is an older game that doesn't have the re-match feature found in later games.
However, there are ways to challenge yourself and engage in battles similar to re-matches:
* Use the "Trainer Replay" function: You can access the "Trainer Replay" function in the game, which allows you to re-watch past battles, but you can't actually fight them again.
* Challenge yourself: You can try to re-battle the gym leader's Pokémon using your own team, either by catching similar Pokémon or by strategizing against the gym leader's team with your current Pokémon.
* Trade with friends: You can trade with friends who have Pokémon that are similar to the gym leader's team and then battle them.
While you can't directly re-battle the gym leaders, you can still enjoy the challenge and replay the game in different ways.