His reasoning, according to Tomlin, was based on his previous 52-yard field goal.
Tomlin noted there was some debate on whether to punt from the Titans' 35-yard line (possibly taking a delay of game penalty to provide a little more room for Drew Butler to pin the Titans at their goal line), to go for it on 4th-and-7 (two big completions had gone to Emmanuel Sanders on the drive) or to let Suisham kick.
Suisham's 52-yard field goal cleared the cross bar by more than two yards, leading to the the confidence he had the leg for it.
The Steelers called timeout, leading to the decision - despite the lobbying of Steelers quarterback Ben Roethlisberger to go for it - to give Suisham a shot at it.
The kick was short but dead center. The defense couldn't make a stop, capping off their third consecutive losing road game in which they allowed 10 or more points in the fourth quarter after taking a lead into the final 15 minutes.
The powers of psychic foresight often elude coaches, despite all fans having it. Hindsight is 20/20 and the kick was clearly short.
What were his options, though? Punting to a short field with an average punter, hoping to get a roll inside the 20 without a return and without a penalty. Going for it with three injured offensive linemen (although right guard Ramon Foster was in the game albeit hurt), requiring tight end Heath Miller to stay in-line in a max protection situation. That also runs the risk of interception under duress, a sack, which could have given the Titans even better field position, or a fumble resulting from a sack.
The play that had the greatest chance of scoring points was the field goal attempt.
It's also fair to dissect a decision to run Baron Batch on 2nd-and-6 for a yard loss. Knowing how depleted the Steelers' offensive line was, and how ineffective Batch is likely to be between the tackles, it (retrospectively) seems wiser to have called a higher percentage passing play with a quick release to gain a few more yards, thus setting up a 3rd-and-manageable. At that point, they have more options. 3rd-and-7 doesn't help them, and an incomplete pass on that down still would have made it 4th-and-6.
And one yard closer for Suisham, assuming a no-gain on third down.