I think they just got stuck in a rut. They have been dealing with an all-obstructionist Republican party for nearly 16 years now, ever since Obama was elected, if not before.
They stopped promising the moon because they became policy wonks and focused on what was realistically achievable, only making promises they thought they could turn into reality with an obstructionist party blocking them.
They stopped being dreamers, started being policy wonks, and were unwilling to make promises they didn't think they could keep. Think about the amount of messaging in the last few elections about how progressives were asking too much because we have to be realistic about what we can pass with only a sliver of a majority. People rightly view that as starting from a point of compromise and thus as weak.
Trump promised to smash norms and ignore laws to get his promises done, which people wrongly view as strong. When Republicans like Trump make promises, they are completely unburdened by whether they can accomplish them or not: make the promise, follow through be damned.
Nobody wants a policy wonk telling them they need to wait until their kids are middle aged for things to get better for their family, and the Democrats somehow failed to realize this in 16 years.
Obama was the last Democrat to run on change in the system. Everyone else has been Bush-era-style "Stay the course" status-quo enabling.
Right, they don't seem to understand the kind of change they've chosen here. If they were actually upset about corruption, they picked the wrong team.