I think there's an argument to be made that many of the allegedly "radical" Democratic policies fall into an uncanny valley of wonkiness, where they're enough of a reach to get people riled up emotionally but not enough to have the kind of punchy, obvious benefits that would get people to be supporting on a similarly gut-level basis. Arguments about whether the minimum wage should be $X or $X+2 seem like accounting tournaments. There's no appetite for saying stuff like "we will seize $100 billion from the wealthiest individuals and give it to everyone else as cash payments".

The other problem is that the Democrats don't seem to realize that incremental change doesn't really work when the system of government is messed up like it is. Every little small-ball policy the Dems try to push through can just be undone later by administrative gimmicks as long as we have the level of ambiguity we do about executive power. Beyond that, they can be rolled back by countervailing legislation because the Republicans are focused on gaming the system. "Substantive" radical policies like universal healthcare are unlikely to be achievable without first enacting "procedural" radical policies like anti-gerrymandering rules or abolishing the senate.