The problem with a primary election in the first place is that it is entirely a matter of picking which candidate you think can motivate more of your base to go to the polls to vote for them, meaning it is literally a contest of who can build the biggest bandwagon, and there is no better way to measure that than to let the candidates all attempt to build bandwagons.

Approval voting would solve the problem; and it would also threaten the party duopoly, so it will never be allowed to happen on any nontrivial scale even though it is trivial to implement.