Petee gave a great summary of all the concrete reasons. I wanted to add that America's government and institutions are widely regarded as illegitimate now by huge segments of the population - particularly people under, say, 40 or so. Ever since 9/11 America has taken hit after hit - the continuous militarization of the police, the government repeatedly and blatantly lying to the population and not even trying to hide it (iraq, stuff from wikileaks), the bailout of the rich and fucking of everyone else over and over (2008 and also during the present economic crash), and evidence of even darker motivations among the oligarchy (fucking Epstein). Every one of these events permanently disconnected some Americans from really ever being able to trust the government or even give it the benefit of the doubt. Although the poor have been struggling for a long time, things were holding together through the 2010s, probably because it was still possible to make money. Now the economy has imploded again, and this one looks worse than 2008. The government's misleading unemployment rate says ~15%; this is bullshit, as that rate has always been. Millions and millions of people are facing eviction and food insecurity, and the President sits on Twitter complaining about how it's all everyone else's fault.
The far right has adopted a militia/siege mindset and views the government as the enemy (despite worshipping cops, which is mainly just due to their confused fetish for power), and their most dangerous fantasies have been stoked daily by the president. They aren't quite as numerous as the media makes it seem, but they are extremely dangerous. Meanwhile, the "far left" is more numerous now than perhaps at any time since the 1930s. Huge numbers of Americans identify as socialist now, and while you can probably argue about whether or not their specific beliefs are really socialist, the point is that they don't view the two-party monopoly as legit and they don't buy into the vague bullshit Democrats come out with anymore. Finally, among the large number of Americans who don't identify with parties or political philosophies, even here there is a more general sense that corruption and decay are dominant. Everyone has a different idea of why or a pet theory they picked up from the internet. And nobody has any political solutions that can actually be implemented because of the rigidity of the Constitution and the two-party oligarchical lock on power.
The lynching of George Floyd was horrific beyond description. It's not my place to say what black people thought of it - many are doing so quite well. It's the latest and most brutal in a series of high-profile police killings of innocent black people, many of whom fear being murdered by agents of the state every day. I'm going to go out on a limb here though and say that this video resonated with many Americans of every ethnicity at a metaphorical level. Floyd was suffocated to death, over a long period of time, for no reason other than sadism, while bystanders clearly explained the situation in multiple ways. The response they got was "This is why you don't do drugs, kids" - the killing was intentional and exemplary. Metaphorically, I see George Floyd as America itself, huge swaths of the American population. We've been suffocated for years. What's happening is out in the open, people have explained the situation again and again, and none of it makes any fucking difference because the people in power apparently enjoy watching us suffer. Biden might have won the nomination, either through DNC fuckery or through right-leaning suburban Democrats and loads of people living in retirement homes afraid of Bernie and higher taxes, but nobody actually likes him or defends any of his policies. "Centrism" as an ideology requires a belief in the systems of government as legitimate and that's utterly dead now. Also everyone's been cooped up in their houses going insane for two months. If there was a Gallup poll right now asking Americans to agree or disagree with the statement "Let it all fucking burn", I would expect "agree" would win easily.
This.
