Is Closed Primary Open To Debate?

Jason Monks of Meridian won a four-person GOP primary in May, collecting 942 votes out of just 2,373 ballots cast. But even as one of the winners in the GOP’s historic closed primary, Monks’ support for the idea is qualified. He told the Statesman editorial board that he was “very frustrated” during door-to-door campaigning in the spring, as he heard from voters who were staying home because of the closed primary. Monks says he could support whatever Republicans choose to do in the future. After some hemming and hawing, he said he’d prefer to keep the new primary system, which requires voters to register with the GOP in order to vote.
Monks’ comments reflect what we’re hearing from local Republican legislators and legislative candidates, when we ask them about the closed primary: mixed and lukewarm feelings/ Kevin Richert , Idaho Statesman. More here.

Question: Why is the Idaho GOP so adamant in its support of the closed-primary when even it’s legislators and candidates are lukewarm to the idea?