Back in the day, zoos were mostly bars and fences (a lot of older zoos still have a lot of fences). Eventually, zoos realized that humans felt better about looking at the animals if there weren’t bars in the way and they started digging pits. Interestingly, this works for real life humans and is the exact opposite for photography. With photography, you want the animal slightly above you, so you feel a sense of connection, even through bars. If the animal is below you, it reinforces the common “humans are better than animals” view, and creates distance. The greater the height, the more the distance and the photos just don’t resonate as much.
That is, unless the animal is interacts directly with the viewer. Then the image can still work.