but how exactly is it that these two are the left's best option?

How has nobody said the obvious: because Trump.

You're asking why somebody competent doesn't stand in front of a freight train or wrestle a bear; it's because they're not an idiot. Only an idiot would run against Trump '24 - and that's what we got.

The politically savvy Democrats see a Trump that's dodged insults, impeachment, indictment, and being shot in the face and say how can I top that? They doubt even having the entire media fluffing them is enough.