Little homerish here, but hear me out...
Perhaps Fred just knows/trusts his guys to make the better decisions, play-to-play and at the end of games, and trusts their conditioning and confidence.
Calling a timeout gives the other team--which might need the blow/the coach talk more than ISU does--just as much or even more than our guys are going to need it.
A rainstorm might slow a run-based football offense down... but it's going to mess with a pass-first team even more. Fred doesn't want to give them the extra help/out?
I think there is some truth to this. I like Fred's style, think some coaches do too much to insert themselves into the game (sometimes their team's style of play requires it), let them play. Of course sometimes a timeout is necessary to make an adjustment or try to stop a run.