Here's why:
* Hidden Abilities weren't implemented until Generation V: The ability system was expanded with the introduction of hidden abilities in Generation V. Before that, Pokémon only had their standard abilities.
* Hidden Abilities were assigned randomly: In Generation V, hidden abilities were randomly assigned to Pokémon, with a low chance of a Pokémon having its hidden ability. There wasn't a specific way to obtain a Pokémon with Wonder Guard, other than getting lucky with a random encounter.
To get Wonder Guard in later games, you would need to:
* Use a Pokémon with Wonder Guard: If you have a Pokémon with Wonder Guard from a later game, you can transfer it to Pokémon Black using the Pokémon Bank and Poké Transporter services.
* Breed: If you have a Pokémon with Wonder Guard, you can breed it to potentially pass the hidden ability down to its offspring.
It's important to note that Wonder Guard is not available in Pokémon Black. You can only get Pokémon with Wonder Guard in games where the ability is implemented, like Pokémon Black 2 and later titles.