The previous answer was pretty disparaging of religion, but one does not need to be. When one begins to compare zoos with religion, there are many things one can see in terms of similarities. Let me give a few:
1. First, let me reverse the logic of the previous answer. Just as man creates zoos, so God creates religion. This is another way to look at things. And it is just as legitimate to say.
2. Zoos can only function because people take care of the animals in them. In a similar fashion, one can argue that religions are able to thrive for so long, because there is reality behind religion. A good example is the explosion of religion in our world. Unlike what social scientists believed in the past about the future of religion, today there is more religious people than ever. Just read any works by Peter Berger.
I think these two points should get you started thinking.
Take the zoo as a metaphor for religion and you may find the exact answer to your question. A man-made institution that holds (with bars/cages/outside control) its inhabitants (animals) indefinitely. Would survival from the cage come from an outside wanderer preaching a new reason of thinking?
Religion is a man-made institution that holds its members inside a particular way of thinking about, dealing with and maintaining one's way of experiencing the world. Would the tiger know there is a river flowing with food, drink and play if he could not see out, never mind walk out of his cage? And furthermore, would he even understand the concept of a river, if he being locked in his cage for the entirety of his life?
This is religion in a metaphor. Would the Christian understand the concept of the river of the tiger or would he know how to achieve any ounce of insight into a term called Nirvana if he was to step outside his cage?