My opinion is that he didn't think he needed it so he didn't include it. A double loop is the easiest thing ever for him (especially when he can do 3L off of practically a standstill). At 2010 Russians he didn't have 3-jump combo. At 2010 Europeans, he didn't attempt a 3-jump combo. At 2009 Cup of Russia, he didn't even attempt 3 combinations, let alone a 3-jump combo. Clearly, he wasn't intending on putting it in his Olympics FS. So either nobody ever told him that he needed it, or because he kept winning without it (and convincingly at that) he never saw it as an issue. Not arrogance, just oversight. I bet nobody ever told him, hey maybe put some more of your jumps in the second half. I'm sure if these oversights had resulted in a loss, he'd surely have implemented them in his 2010 FS and have a 2nd gold medal to show for it, but unfortunately it was at the Olympics that it became apparent that not maximizing his score could actually affect his placement. And he obviously made some efforts to maximize his scoring potential, like putting a quad in the SP and adding spin positions that he normally doesn't do in an effort to increase the levels, but he didn't do everything he could.
If Plushenko competes in Sochi, and wants to have a shot at the podium, he will need 2 quad attempts in his FS, he will need to max out his 3-jump combo, he will need top levels on his spins and footwork, he will need to balance the jumps in both half of his program, including putting difficult jumps in his 2nd half for the bonus. He will also need to increase the difficulty in his choreography between his elements (again, his choreography was 5th and transitions were 7th in his Vancouver FS, which was clean... so clearly the judges aren't going to give him PCS marks just because he skates clean). Even varying the direction in which he skates around the rink is an absurdly simple change that would likely benefit his scores. Unless he can magically transform his skating ability and programs to become on par with those who will likely get higher PCS, he'll need to skate practically clean to beat them. He can't afford to leave out a jumping pass or Zayak his jumps or not maximize his levels or combos. If his performance in 2010 lost to a quadless Lysacek, you can bet that the current crop of men will give him an even greater run for his money. Hopefully he can recover though, because his presence would certainly make the men's competition more interesting.