Why do some political parties in the US have open primaries, but not others?

Question

In the recent California primary elections, any NPP (no party preference) voter was allowed to choose ballots for presidential primaries for American Independent Party, Democratic, and Libertarian. However other parties, most notably the Republican party, would not allow voters who didn't register with them to vote.

I'm not sure I understand why these parties allow non-members to participate in primaries (both as voters and as candidates, e.g.: Bernie Sanders), but more importantly - why the discrepancy between different parties?

In my view it leads to much more extremist candidates from parties with close primaries, while much more centrist candidates from parties with open ones (e.g.: Trump vs. Biden).

Why is that?

Answer 1

Well, Republicans did participate in open primary until 2010, and then they stopped. I wonder if there's any specific reasoning I'm not aware of.

If someone searches enough, there might be a reason/statement in CA too. Anyhow, here's the one for Idaho, which did it at about the same time (2012):

You have to go back five years ago, to June of 2007 to understand how it all began. That’s when the Idaho Republican Party State Central Committee decided to close its primary to only registered Republicans. Jonathan Parker is the Executive Director of the state’s Republican Party. “We do believe that it is our right to essentially let Republicans chose Republican candidates, Democrats choose Democrat candidates, as these are the candidates who will be our standard bearers, carrying the torch for the Republican Party in November.”

TLDR: the official reason is that's how freedom of association should work. (And they won in court, as that story details.) FWTW

Boise State Political Science Professor Gary Moncrief [...] says on a national scale this isn’t unusual - for a faction of either political party to push for a closed primary. “The party that leads that charge is often, in fact almost always, the majority party in the state,” says Moncrief. “In some states it’s the Democratic Party and the faction within the party that tends to lead that charge tends to be the more liberal faction. And in the Republican Party, if they’re the ones leading the charge, it tends to be the more conservative faction.”

I suppose that does make it a bit more unusual for CA Republicans, given that a GOP majority there isn't a fact of life there.

Answer 2

The difference is a matter of state law more than it is a difference due to the preferences of the political party. Each U.S. state has its own political primary/caucus system. Usually, political parties must follow the rules of the state for its nominees to be recognized by state election officials.

The situation in California is purely a product of California election law.

By comparison, in the Presidential Primary in Colorado in 2024, both Republicans and unaffiliated voters were allowed to vote in the GOP Presidential primary.

Answer 3

Question:

Why do some political parties in the US have open primaries, but not others?

As ohwilleke already stated this isn't a political party issue rather it's a state's issue. States run their own elections although they are also run under federal framework of laws and constitutional regulations.

Why have open primaries? The obvious reason is only about 30% of Americans belong to the Republican Party, and about the same percentage belong to the Democratic party. That means 40% of voters are independents. Both parties have incentives in states where elections could be close to appeal to independents and try to get cross over support from moderates in the other party. This formula gives each party the best chances to win elections, state by state.

Why have closed primaries? The problem with the open primary system is, with moderates and independents voting it tends to draw candidates who are also moderates. Closed primaries suggest either the state election isn't really in doubt, or the local state party is more interested in fielding candidates who support a particular ideology than actually getting people elected. Both at times have explained closed primaries across both major parties on a state by state basis.