Everyone else is wrong on the causal relationship of low voter turnout. :D 

A similar thing happens in other states where they hold a presidential caucus followed later by a down-ticket primary. You can see this effect in years where there is no presidential election, in all states.

Drop the caucus in favor of an early primary and people will turn out in greater numbers. Kill the polling station in favor of mail-in ballots and the participation rate will jump. It is well-proven that when you lower the barriers to voting, people will vote.

Or you can believe whatever fits your political ideology, in order to comfort yourself.