To beat themselves would mean that they made a bunch of uncharacteristic mistakes. Can you honestly say that that's what happened? No, the Pats are just a better team. The Steelers had to play over their heads to win, and they didn't. As good as they are, if the Pats play well, it's just not going to happen for them.
No, it's true, the Patriots are a better team. But greatness is a cross between talent and performance. If, say, the Dolphins played lights-out their best game against New England and New England played their best game, could the Dolphins win?
Not unless a bolt of lightning struck Tom Brady dead in the first quarter or the Goodyear blimp crashed into the Patriots' sideline. The disparity in talent is just too great.
Now, if the Steelers had played their best game tonight and the Patriots played their best game, could the Steelers have won? Sure, because the disparity in talent between the two teams is not that great.
The problem for Pittsburgh is that the Steelers have two weaknesses -- their bull**** secondary and the right half of their line -- that are not only a problem when facing good teams, they simultaneously present problems matching up against New England's greatest strength -- it's passing attack -- and negate Pittsburgh's best hope to exploit New England's most vulnerable point -- the middle of it's defense.
To win (and this is relevant because these two teams could meet in the playoffs and/or New England will face this strategy from the Colts), Pittsburgh -- or really any team -- would have to attack the middle of New England's defense with medium passes, draw the secondary in and the DL out to protect its LBs, and keep the Pats' defense off-kilter with alternating runs and long passes.
But the Steelers would have to do this while chewing up a ton of clock and wearing away at New England's defense, scoring touchdowns not field goals in the red zone. Brady and the New England offense is simply too explosive not to expect that they can score a touchdown from anywhere on the field in 2 minutes or less.
But with 2/5, effectively, of an offensive line, it's really difficult to make this game plan hold water for 60 full minutes. And New England's defense doesn't suck -- it's pretty good actually -- it just presents some opportunities in it's middle.
My point is that when talking about good teams it's not just about "playing well," it's about being in the ballpark, talent-wise, and playing to your opponent's weaknesses while minimizing your own.