I think it's less about arbitrary numbers or ratios and more about:
1. For realism builders, replicating the diversity seen in real zoos. Large, charismatic mammals are definitely more likely to be the centerpieces of zoo collections, but they don't actually make up the majority of animals in most zoos (except safari parks). Frontier has done a pretty good job representing these "filler" animals with the glaring exceptions of birds, monkeys, and to a lesser extent tortoises.
2. For all builders, having a diversity of choice between more than one animal of a certain "type". For instance, if I want a spotted Asian pantherine in my zoo, I can pick between the snow leopard, the Amur leopard, and the clouded leopard, which is great. If I want a waterfowl, I have literally one choice, even though every zoo I've been to, even very small ones, has multiple species of waterfowl.
