Why might a writer of creative nonfiction leave out some details?

It is a very good idea to withhold information from the reader. But you do not want to withhold too much. Because the reader could become so confused, they throw the book across the room. What you want to do is withhold just enough information to cause the reader to want to find out more thus to turn more pages. But then in turn, If you tell the reader everything up front why would the reader continue reading the book to the end? They wouldn't need to.