Every Democratic campaign I've been old enough to understand could always be summed up with one sentence: "Vote for us or else the Republican will do evil things". A completely negative message, nothing about why I SHOULD vote for them, just about why I should vote against the Republican.
There's one exception. Obama 2008. This campaign was all about all the good things he was going to do, all the positive change that was going to happen. The word "Change" was so associated with his campaign, it was a meme for years. And Obama won by an absolute landslide.
Now, of course, Obama's actual presidency, he didn't do ANY of that, and instead, was just another war criminal, like every other president. But I do believe a big reason why libs deify him so much is his 2008 campaign, and I think that carried him to victory in 2012 as well. (Even though in 2012, they DID do a bit of that "vote for the Democrat or the Republican will do bad things, and if I was able to see that after they did that with Trump, but applied to fucking Mitt Romney, I would've laughed my ass off).
Now, after Obama was so successful with "change", and the good stuff? We had the complete opposite. Nothing will fundamentally change.
Democrats really hate learning, not just from their mistakes, but from the stuff they did correctly.
They did fear that, but I think in the US in the 30s there was no real consensus among capitalists about the nature of the crisis or what do do about it. Some of them were in denial, many turned to fascism, some tried a coup against FDR. FDR broke through this impasse, chose a direction, and acted, overcoming the opposition within his class. And it was a successful strategy for the capitalists.
In 2008, there was some elite concern and uncertainty due to several structural problems converging, but the ruling class consensus was to make some marginal changes but preserve the status quo as much as possible. Like you indicated, they were not sufficiently concerned to do more than that. And Obama embodied this outlook and implemented this strategy. And it has been a failure for their class and their system.
Edit: I guess my point is, the opportunity existed to force through a variety of possible changes and new directions, but Obama personified how the ruling class did not have the clarity or the will to do anything substantial. And as a working class we were only a nuisance at the margins. There was (objectively) an opportunity for us as well, but we had no organization, no leadership, no strategic clarity, no capacity to act in any meaningful way.