The Raiders aren't a bad team, that's what. The only bad unit on the team is the offensive line, which is pretty banged up. The defense has been really good this year. Top ten run D that gives up some garbage time stats in the passing game.
I'm not just talking about this year though...last year the Steelers were 8-8 and lost to a 4-12 Oakland team, in 2009 the Steelers were 9-7 and lost to a 5-11 Oakland team, and in 2006 the Steelers were 8-8 and lost to a 2-14 Raiders team.