One of the key problems with the US system is that voters are given a binary choice for president which massively oversimplifies the complexity of the decision.
Some social democracies in Europe have a much wider set of parties to choose from though this comes with its own issues of relying on other parties to get policies through.
I think one aspect that has the potential of backfiring on progressive Democrats is exclusion by inclusion. If you can only be non-deplorable if you agree with them on every social issue then it pushes people away. Being a minority doesn’t mean you can’t be sexist, being a woman doesn’t mean you can’t be racist and so on.
Considering how loudly progressive voices carry online, I think if there was a left party, people think it would be able to win. Yet the left candidates aren’t even on course to win the Democratic nomination let alone the presidency.