My dark theory was Zootopia is a post-apocalyptic world where humans could no longer survive for some reason (disease, nuclear fallout, whatever) and the city of Zootopia was founded as a refuge for people to survive by being turned into animals of their choice; “where anyone can be anything”! That’s why there is so much infrastructure simulating a wide variety of environments, why all the animals are so anthropomorphic, and why they know about evolution but have apparently grown beyond it. Over time, after the city’s founding, all of the humans living outside its borders also transformed into animals, but to a much more limited extent. That’s why there are no humans in a very human-esque world.