Political parties aren’t democratic institutions. They are essentially private organizations who can operate however they want. It’s only been the last 50 years or so that either party has used primaries as anything more than a straw poll
You: I want water
The Duopoly: That’s impossible and un-American
They aren't even obligated to donors who donated under the assumption that there's some promise or legal requirement that their primaries be fair. That case was dismissed, and resulted in the quote from DNC lawyer Bruce Spiva:
"You know, again, if you had a charity where somebody said, Hey, I’m gonna take this money and use it for a specific purpose, X, and they pocketed it and stole the money, of course that’s different. But here, where you have a party that’s saying, We’re gonna, you know, choose our standard bearer, and we’re gonna follow these general rules of the road, which we are voluntarily deciding, we could have — and we could have voluntarily decided that, Look, we’re gonna go into back rooms like they used to and smoke cigars and pick the candidate that way. That’s not the way it was done. But they could have. And that would have also been their right, and it would drag the Court well into party politics, internal party politics to answer those questions."
The same sort of democratization happened to British Labour under Ed Miliband, which culminated with the election of Corbyn as leader. In order to fix the problem, they had to purge and expel anyone from party membership that had any sort of firm value system.
Democrats don't have that option in the US, because in the US, people aren't members of parties; they're people who have registered to vote in that party's primary, or people known to have supported that party in the past. US corporate parties have employees, not members. Getting a portion of the public to participate in their primary is the closest thing they have to rallying the membership, and the way that both parties have written election law makes it difficult for them to change anything, or to prevent anyone from voting in them.