I notice you're doing your one sentence per paragraph again.

Ok let's go through this chronologically (not that I agree Biden is conservative, he's center).

Bill Clinton: When you run against an incumbent (Bush senior) you run from the center. So that's what he did.

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 with the Map Room to fight climate change. 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 you're amazed that they don't run an extreme 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.

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 centre to find votes.