At the same time, Nonu wasn't as anonymous last year in Super Rugby as he has been this year. I mean the Blues made it all the way to the semi final's so he couldn't have been playing poorly. He hasn't been helped this year by the Blues playing so poorly, but he certainly hasn't stood out in what has been a terrible team this year.
Secondly, he had been in the 12 jersey for the whole of the previous 4 year cycle leading up to the RWC. For continuity reasons it made sense to make as few changes as possible in the year of the world cup, particularly since the AB's had done so well the previous year. The 3 wise men were well aware of the impact that the lack of continuity had on their team in the previous RWC and they weren't about to make the same mistake again!
Now however, we are at the beginning of a new 4 year cycle, with a (somewhat) new coaching staff. I wouldn't be surprised if there are a few changes.