Because legitimate new characters like Axel Brody, Bomber, Misty would need multiple new and recognisable cards to count as their rewards.
Using Carly as a clone like with Jaden and Joey, and introducing characters that hardly ever dueled like Tristan and Scud means they can make up their own decks from N and R rarity filler cards and save the useful ones for purchasable packs.
Regular Carly actually uses a different archetype than Dark Signer Carly. That's why we get her. Atticus would just end up with another Red-Eyes deck and most of that archetype has already been released yet.
Where's Atticus?
TEZ
