I think it has lot to do with what Fleming ideas are still unused at this point. Because the movie series treated Bond as an unaging superman for so long, the plotlines of Bond dealing with his age, nervous breakdowns, and the imminence of retirement were often cut out of the later Fleming stories when they were adapted for the screen. As a result, those ideas are still sitting around unused. Given how successful going back to Fleming was for CR, it only makes sense that they would scour the well for every last drop. I'm looking forward to Craig playing an aging Bond in his remaining outings. It will be fresh ground for the character and will hopefully allow for the writers to churn out more unique and interesting scripts, even if it means that we will have missed out on seeing Craig's version of Bond in his prime.