As you can tell from the full title, Heilemann and Halperin's book was about the entire 2008 Presidential campaign, not just McCain and Palin. And it's not like the rest of the book was forgettable - the authors told the story of the increasingly hard-edged battle between Obama and Hillary Clinton on the Democratic side, and also revealed the sordid details of John Edwards's tumultuous relationship with his wife and his extramarital affairs, including a child borne out of wedlock.
There's no way all of this could have fit into a two-hour movie, of course, which is why Game Change might have been even better as a miniseries based on the whole book, not just the parts about the filmmakers' political opponents. To ask the question of why this wasn't done is to answer it.
Technical details: the Blu-Ray version of Game Change looks crisp, clear and flawless. If anything, it might look a bit too good for some special-effects sequences, especially when McCain's (apparently computer-generated) campaign jet is in the air. The DTS-HD Master Audio 5.1 surround audio is also terrific, especially during elaborately staged re-creations of the campaign crowds. Every angry word about Obama, in particular, is perfectly clear.
The only special features are a couple of short behind-the-scenes documentaries - no deleted scenes, trailers or filmmakers' commentary here.