Readers have gone through an entire book filled with chapters that have an introductory dialogue between characters. A typical topic of conversation has been Ender and how he is doing; however, chapter 15 has no such dialogue. One reason for an author to change up a regular form or format is to call special attention to it. The shift is meant to be jarring enough that readers notice the change, and we are meant to question why the change has happened or what the change might signify.
Chapter 15 doesn't contain the introductory sequence because the main goal has changed. Throughout the book, humans have been fighting the Buggers and trying to train Ender to be that savior leader. When chapter 15 starts, that goal has been achieved. Ender no longer needs to be trained. He no longer needs to lead; therefore, he no longer needs to be the focus of backroom conversations between Graff and somebody else. Graff has his own personal problems now. Earth has its own set of new problems too, and Ender isn't going to be a part of them because Ender is not going to be allowed home. What's really cool is how chapter 15 feels more focused on Ender without the introductory dialogue. It does this by allowing readers to know that Ender is now much more in control of his final destiny, rather than being a pawn in a military game.