“It is difficult to get a man to understand something, when his salary depends on his not understanding it.”
― Upton Sinclair

It's easy to blame corruption for the reason why the Democratic establishment consistently gets everything wrong. In fact, they get things wrong so consistently that it's hard to imagine it otherwise.
But as George Carlin put it, "It's a big club, and you ain't in it!"
It's natural to forget that corporate groupthink when you aren't part of that group.

Consider what was "common knowledge" not so long ago.

"The lesson of Sanders’s performance may be less about the growing appetite for left-leaning policies than about the unlikely bedfellows that underpin many presidential campaigns."
- Dan Hopkins, fivethirtyeight.com, Jul. 11, 2016

"Why are the Bernie Sanders people so determined to make trouble for the Democrats?"
- Bill Schneider, July 26, 2016

"many Democrats were convinced they didn’t need Ellison to harness grassroots excitement. It’s not just Sanders’s backers who are energized, they argued. And officials said they are confident that most of the Sanders/Ellison wing will come back into the fold"
- Jeff Stein, Vox, Feb 25, 2017

You can almost feel the lack of a clue.
I'm sure that there was universal agreement at those Washington cocktail parties that Sanders' supporters were just ignorant and naive voters that Bernie had misled. They just needed to blow off some steam.
...and they deserved some shaming for not being sufficiently loyal to the party.
After all, no one at those expensive, insider cocktail parties ever admitted to doing anything wrong. So the reason the Dems lost must have been the fault of "them".

It's "common knowledge" that you must go to the center to win elections.
Never mind that for some reason Republicans never do that, so why do they keep winning?
Never mind the horrible record of the centrists.
So what is an establishment Dem do when history proves your basic assumptions wrong?

when this advice does not work, history is retroactively re-written to fit it.

In 2006, the New York Times (3/12/06) claimed that the Democrats lost the 2000 election because Al Gore “unmoored himself” from Bill Clinton’s centrist politics to run as a “populist scourge of Big Oil and Big Healthcare,” and therefore “drastically underperformed.” Yet while he was running, media were presenting him as a thoroughly “pragmatic” politician (Economist, 8/10/00). Indeed, the Times itself reported that Gore’s “centrist agenda” (8/15/00) was so conservative that his support from the Democratic base was wilting (8/17/00, 9/9/00).
...And going further back, both Mondale and Dukakis’ losing presidential bids pushed the Democrats to the right as well. In real time, the New York Times (5/8/88) had praised Dukakis’ “pragmatic, centrist approach.” When Mondale ran, similarly, the Times (7/22/84) depicted his 1984 campaign as “a shift from liberal positions of 1976 and 1980,” noting the only mention of “liberalism” in his platform was to denigrate it. But as FAIR’s Jim Naureckas (Extra!, 9/92) noted, “When the ‘pragmatists’ lose badly with their centrist approach, they are repainted after the fact as radicals, so the strategy of tilting to the right can be tried again and again.”

But then those leftists kept acting out. And organizing.
This was completely unexpected, and that has caused a lot of confusion and tension for the establishment.

A series of tweets by CNN's Jake Tapper revealed that the moderates still don't get it. "House Democrats appeared unified in their votes this week but I've spent the day talking to a bunch of them and many are extremely frustrated," Tapper said. "Other House Democrats are conflicted about having to defend the Squad given things they've said and done. House Dems cited: talk of supporting challengers to incumbent Dems in primaries, AOC's use of the term 'concentration camps,' anti-Semitic comments by Tlaib & Omar." (According to dictionaries, AOC is right. Neither Tlaib nor Omar are on record saying anything anti-Semitic.)

From what I can tell, the difference is a matter of respect. The Dem establishment respects the Republican establishment, so when the Repubs defeat the dems it's all part of the game.
The Dem establishment has absolutely no respect at all for anyone on the left, especially poor people. So when the grassroots fights back, the Dem establishment looks at it like getting bit by your pet dog.

A Times columnist remarked: "It is simply a fact that leftists, as well as the generally disaffected, need to be courted just as moderates do." But the establishment right-wingers in charge of the Democratic Party are not only not courting leftists, they keep actively snubbing and insulting them.

The leftist insurgency can no longer be ignored, and this has caused the arrogant, insular establishment to act out in petulant ways.

On Friday, one senior Democratic source reportedly told Fox News' Brooke Singman the effort would be futile. "No one is afraid of those nerds," the source said. "They don't have the ability to primary anyone."

First of all, they DO have the ability to primary people. Secondly, notice how the media is quoting an anonymous source.
The Democratic establishment is using anonymous sources a lot these days, because they are cowards.

FAIR contributor Adam Johnson and Justice Democrats communications director Waleed Shahid observed, other anonymous sources are not very different in content, because corporate media are generally granting anonymity to sources in the Democratic establishment looking to run opposition talking points against progressive lawmakers and organizations.
...
Another anonymous source in The Hill(7/12/19) branded Ocasio-Cortez as “a puppet” of “elitist white liberals,” in a report on Justice Democrats’ plans to primary incumbent Democrats belonging to the Congressional Black Caucus. The nameless aide went on to say, “It’s offensive that these elitist white liberals feel like they can undermine the foundation of our party.”
Axios (7/14/19) took the unusual step of granting anonymity to a poll, revealing neither who conducted the survey nor who (selectively) revealed its findings—other than “top Democrats.” Writer Mike Allen wrote that the polling group’s name was withheld “because the group has to work with all parts of the party.”