First of all, zoos are no longer freaky side-shows primarily for human entertainment.
Such establishments are an abomination and reputable zoos actively work to get them shut down.
They often end up with the traumatized animals from such places.
They also end up with a lot of other animals that would otherwise be euthanized:
the illegal exotic pet trade
injured wild animals that cannot survive on their own, due to the nature of their injuries
"problem" animals that wildlife agents would otherwise kill (orphaned bear cubs, seals who become habituated to people, etc.)
They also are breeding sanctuaries for endangered species to recover and be sent back out into the wild.
Many critically endangered species only survive because of the breeding programs in zoos, where young are raised until they can survive in the wild on their own.
Also, in order to be a reputable zoo, they must have space, environment, and enrichment that serves the animals' needs.
A zoo environment is never going to be "perfect" just as the wild is never "perfect" either.
But reputable zoos are constantly improving, and getting to where their animals do not feel caged, restless, bored, or depressed.
And, that is what, ironically, caused this sad situation. Gorillas need to be in family groups. Peace is usually kept by the silverback male, who arbitrates the behavior of the family. Somehow, in this case, things got out of hand. It is tragic, but not due to the fact that this family troop was in a zoo. Thousands of baby gorillas have grown up in zoo families without incident.
Please try to be somewhat informed about what a zoo actually is, before passing judgement.
That's all I ask.