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The Steelers aren't the class of the AFC. The Baltimore Ravens are the class of the AFC. Why? See below.
They beat the Steelers. It's that simple. The Steelers are not very good.
That may seem a bit much for a team sitting at 6-3, but I'll break it down for you very succinctly.
The Steelers do not adjust to what teams do well. The sole exception to this is the game against the New England Patriots. That is the only time the team has stepped outside its box to do something different to beat someone.
Against Baltimore, they played their usual zone defense and usual dink and dunk offense. It killed them because Baltimore, of all teams, knows exactly how to defend it. When halftime came, the Steelers changed little, particularly on defense where they were getting gobbled up on third down.
They also aren't very good because they are a mentally weak team. They whither under pressure. The don't play good under the lights. They can't overcome bad calls by the referees (of which this game had many and was, by far, the worst officiated game of the season in my eyes). They can't get up and get motivated to beat a division opponent.
They won't make any noise that matters unless they toughen up and get smarter.