Why is there such an underestimation of the Steeler's Defense?

Okay, First I want to point out that I am in no way shape or form a Pittsburgh fan. I do, however, respect their defense a lot. The Cardinals have just come off playing a top defense in the Philadelphia Eagles, and they handled them easily. However, I feel that after the Ravens and the Steelers, there is a huge drop off until the next best defense. I think that those two defenses are on a whole different level and have played like so throughout the year. though I give the cards credit that their team is playing at another level then they played before, I dont understand why people are automatically saying that Fitzgerald will absolutely destroy them. Yes, he is practically uncoverable, but that defense is smothering and can stop almost any offense. Again, I am in now way a Pitt fan but how are people automatically uming that Kurt Warner and Fitzgerald will steamroll through that Defense?