Radical leftism is the direct result of U.S. having reached its peak in wealth in the post-war period. When economic well-being reaches its peak, the natural progression for a society with democratic foundations is to trend away from rewarding its hardest working citizens, in favor of rewarding those who are "oppressed."
The main reason for that progression is because as the standard of living increases, the amount of value that society places on hard work decreases.
It makes sense if you think about it: As human labor becomes more automated and less time consuming, people have a lot of time to sit on their ass because they can exert a minimal amount of effort and still easily survive. Having not experienced the same back breaking labor and constant risk of death that humanity endured throughout every civilization before us, the average person in today's world has little to no appreciation for meritocracy and the value of striving to earn your keep.
When there is so much wealth and so little effort required to survive, society as a whole naturally places a higher value on psychological warfare than just about anything else.
Why sit in an office for endless hours, when you can be rewarded if you're a "victim" of the "oppressive" boss who thinks you're hot and wants to fuck you? Why put in the "hard work" of raising children with loyal parents, when you can cheat and cash out? Why work at McDonalds when you can sit on your ass and collect unemployment? Why work out for days on end so you can look sexy as fuck, when you can make people feel bad for you for simply being a fat ass who's "unfairly" judged on looks by all those evil attractive people?
When a democracy has reached the peak of its wealth, this is the mentality of your average, stupid, lazy human being: Default victimhood and hypersensitivity.
Without that surplus of wealth, no one would give a fuck about those people or their feelings. But when the wealth exists in abundance (which today it clearly does) then people have way too much time on their hands, so the majority of people in society choose to spend that time on causes that are backed by faulty logic and misplaced nobility.
Logic and nobility are poorly understood concepts for people who are born in the economic height of their respective society. The easier it is to survive, the less of an immediate need people have for logically sound thinking. The less people who's lives are in legitimate danger, the less likely people are to either see or experience true nobility - i.e. "nobility" as it relates to life or death situations, where everything is at risk and thus the nobility of an honorable act cannot be denied.
Instead, the people of today have relegated their immediate "needs" to egotistical bullshit: What other people think about them, and how they can be rewarded so they can be "equal" to those who are better than them, and thus view them as inferiors for practically sound reasons. If everyone's lives were in constant danger because they're poor and need to fight to survive, then everything that society cares about right now simply wouldn't even cross our minds. It would be too stupid to waste our precious time caring about it.
The problem, however, is that humans are emotional beings and typically struggle to invest their time into meaningful pursuits when there is no immediate risk to their lives.
tl;dr Modern day society is at the height of Western civilization's economic well-being. As a result, the vast majority of people are incredibly spoiled, place very little value on their time (because survival is no longer an imminent danger), and thus they choose to spend their time on instant gratification, and are offended by anyone who seeks fulfillment through discipline, hard work, and self-improvement as a means of attaining their goals.
"How fucking selfish! All YOU care about is yourself!"
Modern day people are so spoiled by the ease of survival that they can't even fathom that they're gonna die one day and will never get to come back. So, naturally, they don't value their very own existence anymore than the next person.
How fucking sad is that? People are literally too wealthy to be happy. Without an immediate incentive to be better than the next person, they place zero amount of value on being the best person they can possibly be. "It's what's inside that counts!" has taken precedence over striving to be great, so that one day, deep down we can feel like we're great because we'll actually believe it.
People today are fucking miserable and hate themselves for these very reasons, and thus they default to leftist ideology because misery enjoys company.