There were lots of candidates, but only a few who got significant votes.
Mayor Pete was never a serious candidate (small town mayor? forget the gay issue), same with Beto (had he won the Texas Senate seat and served a term, that would have been a far different matter). Basically, Biden, Amy, Bernie, Warren.
Bloomberg was a rich man's ego trip, but he wasn't even a Democrat.
Problem with Klobuchar is she had some incidents with staff and simply didn't come across well, though wrong year for her, she'd have been a good VP choice in other election cycles. Cory Booker also got screwed by the promise of a black woman, he's more qualified than Harris, probably more than any Democratic candidate (smart as a whip, but from the wrong state).
The Gang of Four, Warren and Bernie got the lion's share of attentions, Pelosi is the other major figure the last year or so (Pelosi has moved toward the center, but SF is hard to shake image wise). The Democrats don't have that group of centralist Governors and Senators outside of the California/NY/Mass axis getting a lot of media attention (Wolf is camera shy). Whitmer has promise, but they need more faces like that on TV. For the rest of the country, NY/California rubs them the wrong way in the same fashion that Texas and Deep South politicians grated on NE liberals. Any Texas Democrat who wins the Governor's seat becomes a viable Presidential candidate for that reason.
Gay marriage is settled b/c the young evangelicals came around.
LBTQ however has been handled poorly at times.
You can't demand that people become tolerant and reject their religion, you can ask that they divorce their personal/religious views from how they treat people in civil society. But you have to pick your fights.
Roe v Wade is a legal, not a political issue, abortion is the political issue, the majority are for legal abortions up to six months, after that they're queasy, and abortion absolutists have alienated potential supporters. The Democrats would benefit from overturning Roe v Wade b/c abortion is a settled issue in the reddest states, but one the Republicans don't want to touch it in the swing states.
Obamacare with a public option is the centralist position, the left of the party wanted single payer and abolish private insurance, a nonstarter politically. The rich will always get better health care (b/c the best doctors can't treat everyone), but minimum health care can be a viable political position (but it will require rationing).
A lot of things poll well until you tell people what they'll pay, whether health care (higher medicare taxes, etc.), climate change (higher gasoline, electricity prices), free college (higher taxes to pay for six year sociology majors).
BLM has widespread support only in a general sympathy sense, as soon as the conversation goes on to reparations, critical race theory and other things supported by the actual BLM movement, it's a no go. Frame it as police reform you garner support, frame as defunding and eliminating the police, no go.
If the nation really reflected those polls you mention, Biden would have won in a landslide. Polls tend to be sloppily worded, and allow people to have their cake and eat it too (I'm for social justice as long as I can continue to give my kids an unfair edge in their well funded suburban school). The American people have always been somewhat "right of center," going back to the Great Depression, where by 1936 there was no longer enthusiasm for a managed economy as the majority had jobs and felt secure, even though unemployment was still 15%.
People aren't in favor of unlimited immigration, they want both better treatment of DACA and more stringent controls on immigration, especially working class people who feel more threatened by increasing the labor supply of non-college educated workers. They don't like harsh measures but if the asylum system is flooded, people will support keeping them in Mexico until they're processed.
The Trump deregulation movement is actually no different than under Bush, Chaney overreached and lost a number of battles, the Trump people got almost nothing done b/c they were amateurs who had no clue how the Administrative Procedures Act works or the legal precedents. But people are split on this, they like deregulation that lowers costs on business but want regulation to protect workers and the environment - and if the poll question doesn't address this contradiction, it will give misleading results.
One trick Democrat politicians need to learn is be strong on aspirations but sketchy on specifics - for one thing, most don't have a clue - listening to Biden on the "Green New Deal' was painful b/c he doesn't understand energy or economics. So don't give a shopping list of programs, provide a general vision of the America you want to see, show your pragmatism by counseling patience, "Rome wasn't built in a day," then say you'll hire the best and brightest to come up with a plan. Our goal will be . . . not I have a plan to do this and that. You're selling hopes and dreams, and dreams tend to fade when you open your eyes.