I don't think it's just a Steelers problem. It's a herd coaching/mediot mentality. The same one that insists Manning is the GOAT. There is no reason for it, it just is.
Look at the Steelers/Saints game. The mediots talked about how great the Steelers were in the first half. Never mentioned they got lit up in the second half. I believe there are two factors.
The Steelers dominate a sizable majority of the league. They don't dominate elite offenses and elite QB's. As such, Brady and Warner and in the past Palmer can light them up but there is no consistent pattern since the next week has them dominating an inferior opponent.
"Pressure" is overrated against elite offenses. Yet the media clings to it like the second coming because it's "sexy". Against elite offenses, you have to mix up defenses, be selective with blitzes and really bring "pressure" up the middle. Not the Steelers forte. They also don't have the personnel to play man and I doubt they can play with the requisite level of disciplined.
Polamalu can't.
However, every system has it's advantages and drawbacks and two recent Super Bowls is nothing to complain.