SO FAR there are basically two theories and those are that the game won't be announced until Greg Keyes is done with the book because the book gives great opportunity for more money.
The second theory is that Fallout New Vegas has yet to be released. The games are published by the same company and are both open world RPGs.
I personally think that both factor are contributing to Pete Hines decision to keep it held back for a while.
We will see an announcement on Game Informer around the time the book is reviled.
We will see a game release shortly after the book comes out.