Democrats have popular support for their policies but cannot for the life of them think strategically.You think they don't understand this stuff? Democrats have been deliberately "failing" to get shit done for 20 years. They keep holding out progressive and populist policies, tantalizingly close to enacting meaningful change -- but nope, somehow it always seems to be a matter of political strategy, badly run campaigns, and bad messaging that derails things.
Surely the Republicans haven't somehow had the better political strategists for decades. They aren't always better funded and they aren't always better organized. And certainly they don't have access to some kind of magic politics potion that the Democrats lack.
The only plausible scenario that I can see is that the Democratic party leadership has no interest in actually getting policy enacted, as long as they keep their cozy niche in power. Another 4 years of Trump will help them do very well in the 2022 midterms, and it will help keep their friends at the NYT, CNN, and MSNBC in business.
It would also explain why they hated Sanders and the DSA candidates so much. Liberals (not to mention leftists) who actually wanted to take power at the national level and actually do something? That would fuck up the whole game.
Basically you'd have to show me some actual evidence that Democratic party leadership actually wants to do anything other than campaign on a progressive platform and consistently almost-win. The whole thing seems worked to me.