From my non-USA perspective I can tell you (from what I have seen and read) that a significant sector of the progressive movement does not see Biden as better than Trump. Biden is opposed to medical care for all, is not against foreign wars, does not want UBI and is backed by corporate lobbysts. Why would someone vote for him if he is opposed to the core issues that they want to see enacted as public policies?

If I were in their shoes I wouldn't vote for him either unless if he commits to at least some of the issues that matter to them.