It's really very simple: If elected, Biden will govern from the center of the Democratic party. And the center has moved left.
"[I]f Biden is elected in November, the left may get a presidency it likes after all — or at least one it hates less than anticipated. The coronavirus outbreak and the resulting massive surge in unemployment has moved American political discourse to the left: Ideas that would have been considered too liberal for most Democrats a few months ago are now being proposed by Republicans. And if American politics is moving left, expect Biden to do the same. Biden was often cast as a centrist or a moderate during the Democratic primaries, but those labels don’t really describe his politics that well — he doesn’t really seem to have any kind of set ideology at all.
Instead, Biden’s long record in public office suggests that he is fairly flexible on policy — shifting his positions to whatever is in the mainstream of the Democratic Party at a given moment. So if Biden wins the presidency and his fellow Democrats are still clamoring for more government spending to help the pandemic recovery, Biden is likely to be a fairly liberal president, no matter how moderate he sounded in the primaries....
It’s hard to measure the precise center of American politics and how it has changed over the last few months. But it’s certainly moved left in response to the COVID-19 crisis — toward way more federal spending....Mirroring the shift in his party, Biden and his advisers are now reimagining his candidacy and presidency — rolling out more liberal policy plans, speaking in increasingly populist terms and joining forces with the most progressive voices in the party. Biden himself has invoked the idea that he might be entering the Oval Office facing a crisis on the scale of the Great Depression."