Because Pandora is about fictional animals, which was always part of the mission. Pandora still isn't a great fit, but Joe Rohde et al. managed to make it work.
Zootopia is about people. It has little to do with actual animals in any way -- they're basically humans dressed up in animal suits dealing with human issues. It fits Animal Kingdom about as well as Disney's Robin Hood would. Even if you ignore all that, I believe the Zootopia they're building in Shanghai is a giant city facade, which they would almost certainly clone. That alone would feel wildly out of place at with the rest of Animal Kingdom.
My larger concern is that if Zootopia works there, then basically anything with an animal (or an anthropomorphic one) should work, which opens Animal Kingdom up to most Disney IPs.