Why does John Knowles write A Separate Peace as a flashback?

The purpose of the flashback is to lend perspective on the
story. Now that Gene is an adult, he revisits Devon and reminisces about his schoolboy days. He
recounts the story from the perspective of his older, wiser self which affords him the distance
to see the incident for what it was and tell it in a more mature and level headed way. If you
take away the flashback, either the story would have to be told in 3rd person, robbing the reader
of the firsthand insight the narrator is able to provide or it would be told by a more emtional
Gene retelling his recent experience in a more emotional and/or less candid
way.