Let's pull up the history, I have this saved because I have to go over it with people:
Let's run through the recent story.
Bill Clinton: After successive losses Bill figured out "it's the economy stupid". And when you run against an incumbent (Bush senior) you run from the center. So that's what he did. And he won.
Gore: After the population hopefully warmed up with Bill Clinton, he stuck his head out left with climate change. And bam he lost the election. Thanks 3rd party protest voters!
Obama: So guess what Obama learned? Don't stick your head out. He ran on vague "hope", hoping the ambiguity would be enough considering Bush's disastrous wars. And he won.
Hillary Clinton: After the population hopefully warmed up with Obama, she stuck her head out just a tiny itty little bit left on climate change (that thing all the leftists care about right?) with the Map Room. And guess what happened? Bam she lost. Thanks protest non-voters!
On to Biden. Just like Obama learned from Gore, Biden learned from Hillary that you don't stick your head out left. And he was running against an incumbent, so once again when you do that you run center. He's actually been governing more from the left, but he ran center.
And people are amazed that they don't run an big left platform? Every time they stick their head out a little itsy bitsy tiny bit left they lose. And the next guy learns to go to the center to win.
Like I said.
So how do you get them to move left? By giving them victories. Consistent and overwhelming victories. Because when they lose, like they've lost 20 years out of the last 24 years, they will go to the center to find votes.