Here's why:
* Shadow Lugia's Catch Rate: Shadow Lugia has an incredibly low catch rate of 3.
* No Other Methods: There are no known methods or glitches in Pokémon XD to significantly increase catch rate.
* Game Design: The game is designed to make Shadow Lugia a challenging and rare catch, and the Master Ball is the only guaranteed way to obtain it.
While you can try to catch Shadow Lugia with other Poké Balls, the chances are extremely low. It's highly recommended to save your Master Ball for this encounter.