They were way too powerful
Dynamax/Gigantamax isn’t just double HP, it also gives Pokémon incredibly powerful Max Moves that can boost your stats. You can use Max Airstream for a speed boost AND high damage. Max Knuckle/Ooze for Atk/SpA boosts WHILE doing good damage. Boosting your stats while using moves higher than 100 power AND being able to live through almost every hit is just too powerful, so they banned Dyanamaxing and Gigantamaxing to AG.