Obama teetered close to the edge of having enough popular support to push major reforms that could have shaken up the whole country because everyone and their dog wanted the real estate bankers thrown in jail. Lock them up, as it were.
Instead we got what we got because they got scared that campaigning on change lead people to expect change might happen. And this time it had the possibility of being more than empty sloganeering: there was genuine popular support that could have carried some of the more serious reforms being offered on campaign. Quite a dangerous thing for the party that pretends to be a steward of status quo, bipartisan consensus, and pragmatic clear mindedness. Obama's charisma lessened the weight of the wet blanket they threw on top of the reform movement.
After him they tried to capitalize on Obamas success to continue the pivot back to safer neoliberal waters. Luckily for them the Republicans had become openly racist because a black man was president and Dems figured it was a safe move. Unfortunately that wasn't enough because then Trump showed up and offered what Obama had: the promise of changing things, pretending to recognize a broken system and offering not just to fix it but to bring reforms to it. Drain the swamp, lock her up, etc.
Trump proceeded to be such a colossal fuckup that they could campaign on "going back to normal" with success. Back to the broken neoliberal capitalism of yesteryear was better than whatever shitshow Trump had done: millions of people were fucking dead, for chrissakes! Anything is better than that!
However they got complacent again. They don't want change. They don't want people to expect change. They want people satisfied with a status quo that erodes little by little in ways the common people can still recognize. But instead of a charismatic orator they had a doddering mummy. People saw that he was, even without dementia, clearly old as fuck. And all they could do was co-opt progressive language to scold those who noticed. Don't do an ableism, Jack! It's just a stutter!
In short they only want to win on their terms. If the electorate refuses those terms? They're quite happy to lose again. After all, the Republican jackals will soon enough remind us stupid proles that we should have accepted their mediocrity. Shame on us, right?