Because there will be a complex publication/marketing schedule agreed across the entire PRH operation to maximise sales, not just media coverage. The marketing plan for this book is driven by the publishers, not Prince Harry or the media, and there are currently no books in the shops for anyone to buy. If the 'best bits' are filleted out ahead of publication, a significant number of people won't bother to buy the book, thinking they've already read the juicy stuff, and what's left will just be a lot of waffling about quinoa and tapping.