There's many factors. But the big one, which is missing in USA is a violent overtly anti-democratic mass movement.
"Missing in the USA"??
Do you live under a rock???
Just stop it. The organised violent and anti-democratic mass movements have no comparissons to the disorganised violent mobs of today. The fascist mass movements were all about extininguishing personal freedoms. To become a willing mindless tool of the govornement. Today's mobs are absolutely obsessed by personal freedoms, to such an extreme degree that it's the pushing of the conflicting personal freedoms is the problem now. Everyone is equally obsessed by democracy. They accuse the other side of not being democratic enough.
Both communism and fascism want to go back to an imagined earlier time where we didn't have choices to make. We would be born into a station in life, where we'd stay, just keep grinding in life, and the state would look after us. The idea of the body politic. All of society to be as of one mind. The very opposite of today's mass movements.
The fact that there's violence today is where the comparisson ends. The type of violence is also different. Mass movements in the early 20th century were organised like armies. The various communist movement in the 1920's and 30's were litterally organised centrally from Moscow. The fascist orginasations where organised like armies.
Today's mass movements are more reminiscent of the Reformation IMHO. or the 17th century. These were technologically driven. Information technology.
The printing press in the 15'th century led to the rise of protestantism, fundamentalism and witch hunts. Because the church no longer could control the narrative disiminated. Kings and princes who wanted to take over Catholic property jumped on the bandwagon.
Industrialisation led to a rise of litteracy and an educated urban class. In the 18th century the aristocrats could no longer use their monopoly of access to wealth to strangle opposition. Because now the commoner industrialists had more money than them and wanted a share of the power. They started printing their own newspapers. Something that was prohibitively expensive before this.
Today the Internet as removed the monopoly of information from the educated urban middle-classes. Previously you had to have access to a newspaper to be able to spread your ideas. Or be able to start a radio channel. To put it in other terms the winner of the 18th and 19th century revolutions were the indstrialist capitalists. It's these people's power that is being challenged today, by the IT billionaires.
What has happened is that issues more important to provincials and the working class have taken over, ie the economy. Rather than virtue signalling about environmentalism, caring about brown people in jungles or whatever marginalised group we're now supposed to pretend we care about.
I see a paradigm shift in politics now. It's the old virtue signalling middle-class urban elites, who are being replaced by a new class of politicians. It's not a left or right thing. The old right is being replaced by a new right. And the old left is being replaced by the new left. And as always when there's a paradigm shift there's social instability and violence.
Calling this fascism is hysterical IMHO
But it is a worrying trend. I agree about that. Let's hope our civilisation doesn't shake itself apart in the process. The only thing I am 100% sure about is that change is comming. There's no going back to the pre-Internet world. That is rapidly dying as those who didn't have the Internet when they were teenagers are litterally dying. Young people care about different things than old people. In paradigm shifts that difference becomes extreme.
edit: Tom Holland has a good quote. "Revolutions happen when people are either out bread or when the newly educated college graduates can't get jobs that match their aspirations". We're on the second one now.