In Colorado, at least, independents get to vote in primaries. In 2016, Colorado Proposition 108 passed allowing independent voters to vote in the primary elections. Now, independents can request a party’s ballot before the election. If they don’t, they’ll be mailed all parties’ ballots but are instructed to fill out just one. If they turn in more than one, their vote is invalidates for all primaries voted in.

Other states could also require laws like this.

Primaries are a unique situation. Very few people vote in primaries. In fact the people who vote in primaries care a great deal about their candidates. The people who care the most about their candidates tend to the far left/right. The end result is one of two outcomes: 1.) the voters in the main election have to pick from a list (two in the U.S.) candidates that are much more left/right wing than the actual voter considers themselves. Or 2.) the party realizes that the candidate that is leading is not generally electable and ignores votes to find someone more appropriate.

I think that the republicans in 2020 followed (1) and the democrates followed (2). This is how we get biden. If a more progressive democrat was the candidate, would trump have won? That was the fear of the democratic party.

(I just made this up so take it for what its worth)