The party stands for nothing. Nothing memorable and stirring, anyway.
Think of the 2016 Democratic Party presidential primary. Only four candidates tried to run, and three of them were dyed-in-the-wool technocrats. The battle boiled down to the technocrat with the highest name recognition, Hillary Clinton, and the upstart populist Bernie Sanders. The party chose the technocrat to go up against the buzzsaw who redefined electoral politics in 2016. In spite of Trump's innumerable (and seemingly fatal) flaws, she lost. She has a lot of excuses but refuses to accept that (1) the race should never have been as close as it was, and (2) the reason it was so close was less Trump's appeal than her own failure to enthuse a lot of people.
Consider the Democratic Party's highest-profile leaders in Congress, Chuck Schumer and Nancy Pelosi. They are defined by nothing except adherence to stale party stances. And they're the ones who, in turn, define the party for everyone else.
The party is run by technocrats who are skilled at the infighting game in Congress. That's okay for governing, but it's hopeless for campaigning.
Now, I'm not dumping on technocrats. I have a technocratic mindset myself. But technocrats are lousy at politics, because politics is as much about emotion as wisdom. You can't run a democracy without emotional appeals because a democracy of any size is full of people who don't know and don't care about any but a tiny handful of the myriad of issues that that democracy faces. You can't reach these people with dry, rational arguments. You also can't reach them with measured hectoring, which is Pelosi's and Schumer's specialty. You have to rouse them with appeals to basic emotions.
Hate, anger and fear are basic emotions, and they seem to work really well with modern Republican voters. However, at least in modern times, the Democratic Party has a poor track record of harnessing these emotions on behalf of its candidates. Lots of people have speculated on the reasons for this dichotomy between the parties; I shall not. I will assert, though, that people are more motivated to vote for a candidate with an inspiring message than for the candidate perceived to be the lesser of two evils. Given the Dems' obvious inability to marshal the more negative emotions anywhere near as effectively as Republicans, Democrats must find a message people can rally around, a message more inspirational than "We're not as bad!"