He’s spoken before about how he was under a lot of pressure to have his be the first free-market explanation of the crash on the book shelves. I imagine he just didn’t have a whole lot of time to go into side subjects (like the GD) in that much detail.
I also could see his interest in (a) wanting to keep the book relatively short, so as to garner more readers, and keep laymen interested, and (b) not get bogged down in the history of other economic events that not only have been covered elsewhere, but also don’t have anything to do with the 2000s boom/bust, which was the main subject of the book.