It was still fairly early in the game, and they would have been insane not to just get the field goal to tie it up before the end of the first half.
Roethlisberger also stated (and so did coach Cowher) that they would have gone for the touchdown on 4rth down if the call went the other way. From the 1 inch line, their chances were excellent either way. Oh, and from the many replays I saw, I believe the tip of the ball did cross into the white line, but not over. Still counts as 6. All that is needed is for any part of the ball to cross the plane of the white line.
ping !