Right now, 83 % of congressional districts in the united states are either very blue or very red. That's why you have a re election rate for individual members of 92%. Even while congress has a national approval rating of only 28 per cent. If you change from close party primaries to open primaries, where anyone can vote, your electorate shifts from hyper partisans to the general public. And your incentives to become reasonable and compromise ind trying to deliver something for your constituents shoot up.