Why Jake Chapman lost when Iowa Senate Republicans saw gains

Iowa Senate District 14 is a winnable seat with the right candidate and campaign.

Iowa Senate Republicans on Election Night saw their already substantial majority expand to a 34 to 16 supermajority, picking up two seats. The same evening, their Senate President, State Senator Jake Chapman, R-Adel, lost his re-election bid, the only Republican incumbent in the Iowa Senate to lose. 

State Senator Sarah Trone Garriott, D-West Des Moines, defeated Chapman by 873 votes out of 29,346 votes cast. She moved to the new district (that included parts of her old district) to avoid a contested primary with State Senator Claire Celsi, D-West Moines, when they both were drawn into the same district. 

Chapman had a voter registration edge of almost 2,000 voters, but 13,359 independent registered voters are in the district. 

Why did Chapman lose?