Looking at the turnout figures… in Iowa it looks like it was just above 50% who voted for US Senator ( 1,092,734 / 2,142,304 = 51%) with 1747 of 1781 Precincts Reported.

In the 2012 general election the turnout was 73.3% .

So there is your difference.

We can whine all we want to about the backwardness of the GOP, but apparently a significant fraction of your fellow citizens don’t think that is important.