They had already tried a 4th and short and got stuffed. They were going to need to outscore the pats by 22 points. Taking a field goal was one way to make it happen.
It's always easy for us fans to think that we know better than the guys doing the jobs. no?
I still say the Steelers had the momentum to make a TD on the 4th down. You could feel it in the air. It woulda been worth it, to TRY. Now they will never know if they could've done it.