I think Schmidt said on one of the podcasts that statistically you have a better shot of scoring a touchdown on either offense OR defense (if the ball is turned over on downs) when deep in the opponent's territory, so I think that's why they eschewed the FG in that situation. I'd say it paid off, too, since ISU had to punt out of the end zone, yielding excellent field position for UND's next offensive possession, which turned into a touchdown.