Here's why:
* Regigigas is unlocked by capturing all three Regis (Regirock, Regice, and Registeel) in the same save file. This means you need to catch them yourself, not trade them.
* The GTS is a trading system. It allows players to exchange Pokémon with others. It does not affect the game's internal requirements for unlocking specific Pokémon like Regigigas.
Therefore, even if you obtain all three Regis through trades, you will still need to catch them again yourself to unlock Regigigas. This requirement applies to both the original Ruby, Sapphire, and Emerald games as well as ORAS.