Personally, I think that the election of Barack Obama emboldened the extreme left to want their full agenda now, and anything less than that is not acceptable. Gone are the days where they would have to couch their positions in more reasonable-sounding terms. Add to this the election of Donald Trump being a major setback to their extreme goals, they went full-on vicious until they got rid of this impediment.
Joe Biden seems to correctly read the political direction of his party. He has proposed an additional 6 trillion spending over the normal 4.5 trillion dollar budget. Never mind the fact that the government only takes in about 3 to 3.5 trillion in taxes.
Let's say that Joe Biden gets everything he has asked for. Then what? Are the people on the extreme left going to be satisfied with that? Will they say that we have spent way too much money and just for the sake of fiscal responsibility we need to cut back? No, they will say that this is only the downpayment.