Why doesn't John tell all of his people the truth about who the "gods" were in "By the Waters of Babylon"?

John does not tell his people about the gods of New York being regular men and women because his father tells him not to. 
John's father is not against the people eventually knowing about the Place of the Gods being inhabited by humans. He is against the idea of John telling all of the truth in one moment.

After that, I wished to tell all the people but he showed me otherwise. He said, "Truth is a hard deer to hunt. If you eat too much truth at once, you may die of the truth. It was not idly that our fathers forbade the Dead Places." He was right—it is better the truth should come little by little. I have learned that, being a priest. Perhaps, in the old days, they ate knowledge too fast.

John's father, the head priest, is afraid the new knowledge will hurt his people. They have a certain belief system in place. That belief system guides their actions, laws, and behaviors. It's the entire basis of their society. If John were to tell the entire truth, it would be too much to handle. The people would realize everything they grew up to know is false. It would be devastating news to many people. As John says in the above quote, "eating" too much knowledge too quickly is dangerous. John and his father agree that the knowledge should come out, but they believe in doing it little by little.