Why the Democrats lost is a no-brainer: The rank-and-file cannot discern the Party's priorities and intentionality. The Party must have an agenda that its voters can rally behind, and it must show its members reference points that give the Party a place in the future of the country; but the Party can't do that. It can only show different entities within the Party pushing different agendae; so a lot of Democrats did not vote.
How far should the Party define itself as "Left", i.e. when it wants to redistribute a citizen's wealth? Many Democrats also possess wealth and don't want to give it up, unless they have a good reason to. Should Democrats empower the government to make itself responsible for every citizen's well-being? Can it also respect his space and allow him to take risks?
Perhaps the greatest problem for Democrats' self-definition is the dichotomy between written texts published by the Party that describe its policy-initiatives and the ways-and-means, and the images that appear in social-media sites like Facebook. Their coarse, low-brow simple-mindedness do not connect to the Party's intentions. They have the mentality of revolutionary underlings.
Do Democrats define themselves as "Socialist", i.e. wanting communal ownership of the means of production? Do they want to legally end Capitalism, or at least limit the ability of people to become wealthy? Asked in a more personal way, do they resent the influence of Elon Musk, Jeff Bezos, and others like them?
How far do they define themselves as Marxist? Do they support a so-called "collective" approach to policy-making and fostering a group-think ethos, which inevitably limits the freedom and initiative of the individual? You can't talk about "collective" without advocating government measures to pull the wealthy down from their height and using expropriated wealth to push the poor upward.
The average Democrat sneaks over to Facebook to view posts from left-wing bloggers and to enjoy the crass browbeating of influential Americans, with suggestions of vengeance wished against the ultra-wealthy. All this makes Democrats ponder their future in a negative light, even if they derive pleasure from, and even identify with, the bloggers' intentions.
Facebook is essentially an anti-American forum that views America's strengths in a fascist, racist, and sexist cast. The posts convey that America must go to the wall to expiate its sins. Many of the bloggers represent themselves as "Anarchist", "Communist", or other nut-jobs. Most of them use a "handle", rather than their real names. It adds to the air of mystery and danger of Facebook, since no one knows who they are. Rank-and-file Democrats have mixed feelings about having to share space with anonymous bloggers and resent their leaders letting the bloggers influence the Party's agenda. They show no particular loyalty to the Democratic Party, nor to representative democracy, itself.
In 1920, an anarchist conspiracy in Massachusetts killed two men in an armed robbery. The robbers shot one man four times and the other in the back as he attempted to flee. They "were members of anarchist cells that espoused violence, including assassination" (according to Wikipedia). The state of Massachusetts executed them seven years later, after their appeals ran out. The other conspirators fled back to Italy, which had its own problems with Anarchism.
Various leftists in the years following the trial and execution tried to exonerate Sacco and Vanzetti, even as the historical evidence mounted that supports the verdict. No group has tried harder than the Communist Party. How far "Left" do you have to go to support Sacco, Vanzetti, and the violence of the anarchists?
The point I make is this: If the Democratic Party cannot differentiate itself from the Anarchists and Marxism, or if it cannot distance itself from the violent intentions of Facebook bloggers who want to feed Musk and Bezos to killer whales, or guillotine even modestly wealthy Americans, then the Party will turn off the people who don't want to go that far with retribution.