It'll be harder for a leftist candidate to succeed in the Democratic Party than for a proto-fascist to succeed in the Republican Party, yes. But that just speaks to the strength of the party's resistance; I'm pointing out factors that define the party's ability to resist. Even if everyone really wants to keep an outsider out, they still need some way of coordinating opposition, of getting opponents on board with something as drastic as dropping out, they need to be facing an outsider who isn't strong enough to bulldoze their way through that opposition, and they need to get a bit lucky. They won't always be able to do all of that, even if they're strongly opposed to the outsider.