Ok let’s go through this chronologically.

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! Aka the left never shows up.

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.

More on Obama: so he enacted ACA. That’s great, right? The thanks Obama got for that was to lose the house of representatives for year 3 and 4. Then lost the House of reps again for years 5 and 6. Then he lost both the House of reps and the Senate for years 7 and 8. Thanks voters that can’t be assed to show up after the first election! Aka: the left never shows up.

Hillary Clinton: So what did Hilary learn from the last 6 years of Obama? She learned that the left never shows up. So she ran a mostly center platform, with a nod to left on climate change (that thing all the leftists care about right?). And guess what happened? Bam she lost. Thanks protest non-voters! Aka: the left never shows up.

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 a big left platform? Every time they stick their head 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 first. 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.

With this history, you’d be an absolute fool to cater to the left. Because they never show up.

(Edit)Why do leftists need to move right

They don’t need to move right. They need to show up and give dems consistent and overwhelming victories first. Show the dems that they can win without having to go further and further to the center. Because every time the dems lose, because the left never shows up, they will go to the center to find voters. Voters which BTW actually do show up!

Center voters are worth double, because it’s a vote you get and vote the other party loses. They aren’t going to cater to the left (worth half the vote) when history shows time and time again that they never show up.