The made several stupid mistakes and the Jags simply did not. The Steelers would make a stand but an incredibly stupid penalty would keep the drive alive, things like that. Yes, there was the missed tackle that resulted in the Jags only TD, and a few missed stops that keep Jags drives alive, but for the most part the D did their job. Granted you have to give them credit for the fact that this has been the longest they've been on the field in a LONG time. The Jags played the role of the Steelers in terms of TOP, keeping hte Steeler D on the field for nearly 35 minutes - usually its the OTHER teams' D that is on hte field for that much time if not longer. So their D is on the field a full 10 minutes longer than they are used to, 35 compared to 25 and it sounds like you are suggesting it was mostly a D issue?
If anything the D stepped up and proved it could play if it was kept on the field and made some stupid mistakes. More time on the field = more tired D and more mistake prone. If you ask me it was an all around team effort. After the Jags first TD the Steelers D didn't give up another TD and their O scored enough to win. Ben stepped up and directed the offense through the air when they needed it most. If anything I'd blame the close game on the lack of their run dominating OFFENSE. Props the Jags coaching staff for knowing how to play the Steelers and stick to it - Taylor only averaged 2.8 yards per carry (there was a point in the game when he was 20 rushes for 28 yards, a measly 1.9 a carry) but they controlled the ball and the clock, for some reason the Steelers got away from their grinding run game that is the key to controlling hte clock. Heck, Staley averaged 3 yards a carry, and Bettis averaged close to 6, it seems they easily could have done so but they did not, perhaps it had a lot to do with this being Staley's first game and he just wasn't 100%, perhaps it had a lot to do with them trying to mix in Bettis too much and throwing their own offense off beat just enough, I dunno.
Luckily Ben didn't have as much of an "off" game as he had in the some of the past few games.