In most countries you need to be a party member to engage in internal party politics. The idea that the heneral public makes direct choices for private political organizations is, honestly, kind of weird.
But also, which states require you to be an actual card-carrying member to participate in the primary? I was under the impression that most merely required that you register with the electoral office as a party supporter.
Being a "registered X" is very different from being "a member of X". Members get to do things like go to convemtions where party policy is discussed and voted on. Members get to vie for party nomination. They're part of the internal machinery of the party.
Yhey're not just voters with a party banner.
Well, further end, at least. There's been very little in the way of movement leftward in the last 40 years.
Giving a sliver of a shit about PoC and vulnerable minorities is not a shift leftward.