Why does Tim O'Brien organize his story the way he does?

The book is stylistically unique not only because of its sui generis status regarding fiction/non-fiction and novel/short stories, but also because of its voice. There is only one “I,” the first-person narration of the fictional Tim O’Brien. But an omniscient narration gets into the minds of many of the other characters as well, and we only read metaphors they would think, only have access to words they would know.