Because you cast whatever creatures Azula swipes, you don't need to worry about giving it back if your opponent removes Azula like you would with Mind Control variants, and the enters ability means your opponents are always two-for-one'd at best
Well, it's because you can easily slot a few Mutavaults into your list - an MTG land that can become all creature types - to supercharge the card
This isn't to say that Nadu also didn't deserve a ban due to it's power level, as it was a top deck played at the highest level of competitive Commander (aka CEDH)
Starmie, Gyarados, Blastoise, Articuno, and Palkia have all had genuinely viable EX versions, while Greninja is useful across several decks, and Vaporeon, Manaphy, and of course Misty have each been added to provide very strong, Water-specific support in different forms