Shipped to many jobs overseas. Rich get richer even though they just piss the money away. Automation, anti union fanaticism. Hatred for populism. Unions sold out or were defenseless against corruption.
I think the answer is partly because of Vilfredo Pareto, and his well known Pareto Principle, i.e. the 80/20 rule of some systems. Applying this to socioeconomics we have 20% of the US population making 80% of the money, so now 80% of us are fighting it out for the remaining 20% of what's left over. While I was working a retail job in my early 20s my boss, a short heavy set Russian named Dave came up to me once and asked me, "You know why 20% of people make 80% of money? I said, "No." He said, "Because make money very hard." Then he slapped me on the back of the head and said, "Get back work." I also heard/read somewhere, maybe here on ET, that if you were to take all of the money in the world and evenly distribute it across the entire population giving everyone an equal economic start, that in a very short period of time say 5-10 years you would wind up with 20% of the people controlling or earning 80% of the money. IOW right back to where we are today. From what I have experienced in my 59 years of life, living and competing here on good ol' planet Earth I believe that this is true. It's just the way humans are.
Actually the top 20% only make 53% of the income. Perhaps you are thinking of wealth, where the top 10% own 72% of the total wealth. https://www.statista.com/statistics/203247/shares-of-household-income-of-quintiles-in-the-us/#:~:text=About 52.7 percent of the,20 percent of the workers.
Complacency is the biggest reason for poverty in the States. People need to work hard the same way the people did 200 years ago when they built America.
Main reaaon for economic poverty in USA ? - https://www.dogpile.com/serp?q=Main+reaaon+for+economic+poverty+in+USA
There is no particular reason. Everywhere in the world is the same before the pyramid. Human and animal world are structured the same - alphas and providers rule. Don't feel sad if you are not on the top, nor feel bad if you are in poverty.