Nick finally tells us the truth about Gatsby this late in the book because if he had done it earlier in the book we would have a totally different opinion of him. It is important for the mood of the book that there is a certain mystery about Gatsby, its not till he reunites with Daisy do we find out the truth. Nick saves it for this time because if we knew everything about Gatsby from the start of the book then we would know that he is “new money” and the perspective on people like that in this book isn’t good. That’s bad because Gatsby isn’t really a bad man at all.