Your kicker makes a 46-yard FG earlier in the game like it was nothing. From the ~5-yard line, you don't trust him to make another.
It's easy to criticize the decision in hindsight, but it really wasn't that bad a call. They'd been shredding the Seahawks defense with the run that entire quarter, and if you get that first down, the game is 100% over. Making a FG would have given them an 8 point lead, which again, in hindsight, knowing that the 'Hawks didn't covert the 2pt conversion, would have been enough....but Zimmer also knows the Russ is capable of converting those types of situations, and it would be preferable not to go into overtime, where you gotta let a coinflip decide the game.