Can someone explain this to me?

Ok, early in the 3rd quarter of the super bowl, the Packers were leading 21-10. The Steelers then scored a touchdown, bringing the score to 21-16. The Steelers proceeded to attempt a single-point conversion, which they compleated, bringing the score to 21-17. Now, correct me if I'm wrong, but I think it was stupid of whoever made the desision not to attempt a two point conversion. If successful, it would have made the score 21-18, meaning that a feild goal later in the game for the Steelers could tie the score. If the 2 point conversion was not successful, the score would have remained 21-16, which is no worse then a score of 21-17 because a subsequent touchdown for the Steelers would result in them taking the lead either way, and a feild goal would still result in them trailing by 1 or 2 points respectively. I don't see how the single point conversion was the better choice. Can someone explain if there is something I'm not getting here?