I honestly think it's because he's checked out. If the car was capable of winning a race I think he'd raise his performance another level. You see it with a lot of the greats, they need something to motivate them to the next level, even if it's subconscious. Reminds me of the Chicago Bulls documentary (last dance?) where Michael Jordan admitted to making something up about someone insulting him in order to motivate himself to dig deep and go again.
It's all conjecture, but we've seen it Lewis before, and in other sportsman, where unless they have a chance of winning, they can't (won't?) perform at their best. I find the psychology of sport fascinating.