Dems were constantly spitting out their bold plans to make things better, it's just too many people didn't pay any bloody attention and the media refused to cover it. Fuck the forth estate, and all that.
The party was sure, but Kamala Harris did basically no outreach, one debate vs Trump but no debates vs other Democrats, no unscripted media interviews or unscripted dialogues with normal people. Whereas Trump beat out all the Republican candidates, was constantly touring USA getting free publicity stunts, doing unscripted media interviews with just about anyone (and unscripted interviews in non-traditional venues like McDonalds, Elon Musk, even youtube influencers like Logan Paul).
The lack of clarity and genuineness vs other candidate's overabundance of it is bad for that kind of voter engagement, when voters aren't clear at all which side of the fence you're gonna sit on, whereas Trump leaves nothing to imagination with his rhetoric
Unironically think Bernie would have rinsed Trump if he was the candidate because voters would've had something to attach their imagination to. When we had Jeremy Corbyn vs Boris Johnson, Jeremy did this middle of the road fence sitting where he wouldn't declare if we was going to be liberal or socialist. Boris made promises to the moon that he had no chance of fulfilling - but he still made them, and people could understand the vibe
Trump picks big visceral imagery like building walls or blaming immigrants for eating pet dogs, and it spreads faster than his broken promises or fact checks. Kamala had 4 years to make her economic platform clear and people still didn't really understand what she was doing, if anything, to help people who can't afford their own political lobbyist