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.
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.