Why is there no introductory dialogue for chapter 15 in Ender's Game?

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.

Why is there no introductory dialogue in the final chapter of Ender's Game?

Orson Scott Card begins each chapter of Ender's Game with the text in italics in order to alert the reader that the conversation happening is not taking place between two people in the same room.  The conversations are always about Ender and being done via a radio communication of some sort.  In the world of Ender's Game it might not be a radio communication though.  The exchange is probably being done via "ansible" which allows instantaneous transmissions regardless of the distance between two speakers.  Radio communication is fast, but when dealing with distances in space, light speed is just not fast enough.  Chapter 15 doesn't need to use the technique, because the conversation is happening in real time between two men that are physically located within the same room.  Card uses regular dialogue formatting to indicate that fact. 

Card is not unique in his use of italics at the beginning of a chapter to alert readers to a different kind of narration.  Author John Krakauer makes frequent use of this technique.  He doesn't use it for dialogue like Card does, but he uses it to incorporate words of other writers that help him develop that particular chapter's point.  The Night Circus by Erin Morgenstern uses a similar technique as well, although hers happens less frequently and the different formatting functions as a complete chapter that often signals to the reader a significant passage of time.   

The previous conversations all take place via the ansible, or through some version of its technology so that real-time conversations can take place across a vast distance.  They involve various people who make decisions regarding Ender's training program particularly Graff.

In the final chapter, there is no longer anyone governing Ender's future or deciding on it and the conversation with Graff is one in the present, the physical present, so there is no longer any need for the representation of it the way it was in earlier chapters.