It's not a secret that Iowa is ranked at the bottom of the barrel in the NCAA for possessions per game (less than 58 poss per game). In doing this, Iowa slows down the game and the tempo suffers, as well (only Denver, yes that's a college, has a slower adjusted tempo than Iowa and the Hawkeyes actually have the claim for the slowest raw tempo in college basketball). So right there you have significant statistics to prove that Iowa's opponents just aren't getting enough touches to allow them to score many points.
Even more data to back up the thought that Iowa's good defense is a misnomer. Iowa ranks 261st in steal% and 261st in block%. So Iowa isn't creating turnovers at all.
Also, Iowa allows opponents to shoot over 34% from behind the arc (213th worst in the nation).
Iowa is aggressive on the defensive side of the ball and I completely appreciate the effort they put on the court. But while some seem to think that defense is keeping Iowa in ball games, it's more likely it's Iowa's offense (or lack thereof) that is the real determinant.