The reason is because I don't think that the Cardinals o-line can handle the Steelers d-line. If the Steelers can get pressure on Warner, they will have a hard time getting the ball to their WR's. Plus, I think the Steelers are the more physical team.
The other reason is that the Steelers have Super Bowl experience.
Now, that said, I will be pulling for the Cardinals, of course! Maybe they will surprise me and run some trick plays and pull the thing off. Maybe the Steelers will get cocky. Who knows?