Because this is what changed this election. The progressive voter base of the Democrats has made clear that they are willing to not vote, instead of voting for Biden. The Democrats were happy that the Republicans moved right, as they could occupy the center and win most elections. This forces them to actually move further left to appease the progressive no longer base and win those votes.

Well done. This is actually a good way of moving the country in the left direction. However they also have to show, that they are willing to vote, if the Democrats do what they want or at least move towards them.