California did that to keep Rs off the general election ballots. They figured out long ago (around the time they put forth prop 14) that if a county has a 60-40 R-D split, they simply need to make sure a dozen R's split all the R votes and only two D's run in the primary. That way, even though 60% of the votes were for Rs, the two Ds got the most individual votes and will both move on to the general.
The problem with that would be that a Party could not be sure that its primaries were representative of their voters.
Parties are the scourge of the system.
California did that to keep Rs off the general election ballots. They figured out long ago (around the time they put forth prop 14) that if a county has a 60-40 R-D split, they simply need to make sure a dozen R's split all the R votes and only two D's run in the primary. That way, even though 60% of the votes were for Rs, the two Ds got the most individual votes and will both move on to the general.