I'm guessing that Raji played 2013 completely within the system, actually doing what he was asked: to double team and keep the ILBs clean. He didn't freelance to penetrate because he was focused on doing what he was supposed to do.
It just so happened that his low statistical productivity made it look like he was in severe decline, and no one wanted him. I think he was really just supremely miscast as a DE, but he was still a better DE than Pickett.
So hopefully, as a NT, he'll start showing up on the stat sheets more.