• It’s not just about pledged delegates

    The leadership of the DNC, DCCC, DSCC, etc., are chosen by election, by members of each committee. State parties send their delegates to participate in these things.

    despite not being an incumbent

    Yeah, that’s what I’m talking about. These are processes that longtime party members participate in, and run on, about the structural rules and procedures to follow, and they’re open to everyone. Elections often pit “establishment”/“insider” candidates against “insurgent”/“outsider” candidates, and there are examples of each kind (or hybrid candidates) winning the nomination in the modern primary system.

    It’s more of a spurious correlation: incumbency doesn’t buy the advantage in the nomination race, but reflects that a candidate has the network and resources to have the popular support of their own party. That’s why incumbents always win the nomination, and tend to win reelection in the general.