I didn't press for the reasons why, but tradition has long been that the reigning continuation author does the novelizations, and at the moment there is no reigning continuation author. IFP is now contracting authors one book at a time, as we've seen with Sebastian Faulks (Devil May Care) and Jeffery Deaver (Carte Blanche).
So it looks like we will be experiencing Skyfall on the screen and not the page. But I'm sure it will still be good. ;)
Thanks to the always awesome Corinne Turner for giving me the scoop.