1) It's much easier on the offense when they aren't dealing with the crowd noise. Communication is easier and the OL gets off the ball faster when crowd noise isn't a factor. Likewise, the QB's cadence doesn't have as much of an effect on the DL. The offenses advantage in this area is negated which can make 7 step drops more difficult to execute because of protection issues. This doesn't mean you shouldn't call downfield passes at all, but it should at least be noted that it makes things more difficult.
2) Ohio State has better skill players, specifically on the outside. Not to say that Iowa doesn't have players on offense that are matchup problems (Fant, Wadley), but Ohio State has a few more weapons that can specifically challenge this offense. In order to combat this on defense, Wisconsin hasn't been quite as aggressive, quite as often as normal. They can't afford to put their guys into too many bad situations in the defensive backfield. What we've seen is some 3 man rushes from time to time and Barrett has had loads of time.
3) Barrett is a threat to run. I know a lot of people would love to see Iowa get more dual threat guys at QB (myself included), but Iowa doesn't have that. Ohio State can call plays downfield because they don't have two freshman OT's and Barrett can potentially bail out the offensive play call if the protection doesn't allow him to wait for something to open up downfield.
4) The game is played in the dome. Temperature and wind are non factors in this game.