Pretty much this.  I was disappointed MN didn't go for it as well, but that wasn't the difference.  The D played well early in the game, and with the weather there was reason to believe this would be a really low scoring game (and in fact, it was only 10-7 at the half).  Field position matters in a game like that, so the staff took a calculated risk.  Probably wrong, yes, but if you get stuffed there on 4th would we see this outrage?  The way MN ran the ball early, there was a good shot it gets stuffed.  
Wisconsin had a great gameplan.  They schemed up 4 or 5 wrinkles on O to get matchups and EVERY SINGLE ONE of them worked to perfection.  They schemed up a D to get pressure up the middle while pressing our WRs and it worked 85% of the time and really rattled Morgan.  Pretty much the worst game imaginable.  Sucks.