Why does Fitzgerald suddenly stop Gatsby from having his dream come true just when it seems it will?
This is a good question. There are at least two reasons Gatsby was destined to fail. On a superficial level, the book is about the love of Gatsby for Daisy. However, the deeper theme of the book is the corruption of the American dream. The decadence of wealth and the pursuit of pleasure in excess have destroyed a sense of propriety. All of this would lead to the Great Depression in 1929, when there would be a reversal. Money cannot buy happiness is one of the undercurrents of the book.
Second, Fitzgerald also paints a picture of the hollowness of the upper class. It is society that is obsessed with externals. In this context, seeming is better than being. The newly rich are gaudy and downright silly. The old moneyed are better in their tastes, but they have no heart and sense of morality.
When we put these two themes together, there is no way that Gatsby could achieve his dream. From the start, he was on precarious ground.