The Colts are not built to stop the run, they never have been. This year though with all the problems on offense it affects the defense too. The Steelers need to establish the run early and often. It takes the pressure off of Big Ben, who's probably going to be sore heading into that game after taking a shot on the knees by Seahawks DE Raheem Brock. It also takes the pressure off the offensive line and the pass rush that is sure to come in primetime by the Colts front 7. Getting both Mendenhall and Redman in the game early will also help take the crowd out of the game.
Given the offensive woes of the Colts so far, the defense ends up being on the field longer, and they have no confidence in the offense to make a play when they do give them the ball. If I'm Bruce Arians, regardless of how much I want to throw the ball, you bust out the power run game and really just ram it down their throats, especially early on.
That slows down the pass rush as I mentioned, and forces them to commit more guys to stop the run and then you open up all the stuff downfield with the Steeler wide receivers. Not to mention the fact that you wear them out and demoralize them which will pay dividends late in the game. The passing yards will come still, but for this game, it's a great time to bust out the run.