From 1850s until 1940s, extremely wealthy figures such as Rockefeller, Carnegie, Ford, Vanderbilt, Astor, Mellon, JP Morgan etc. lived, having fortunes over 100 billion 2017 dollars. But after the death of Henry Ford in 1947, Haroldson Lafayette Hunt, Jr. became the wealthiest man in America, with a net worth of only around 5 billion 2017 dollars in it's peak, and we don't get a mega-rich person until mid-1990s with Bill Gates.(by mega-rich, I mean comparable to the men I described earlier) Today, there are 45 people in America with a net worth over 10 billion dollars. What caused this shift, and what reversed it in 90s/2000s?
Differing tax rates played a part. Its discussion is obviously particularly difficult because of peoples' strong feelings on the subject, but it's not hard to draw conclusions when you look at how the tax codes have changed over time.
There are obvious extenuating factors like inflation, new types of products and services, and changes to taxable revenues. To fit your timeframe, the highest bracket for filing federal income tax in 1940 was for those earning $81 million (inflation-adjusted) and up and was taxed for 79% of income versus 79% for $800k in 1980 and 28% for everyone over $57k in 1990 (few people remember we had a bipartisan federal flat tax experiment in exchange for other hikes that didn't go particularly well). There were also many more brackets back then, which has fluctuated but overall reduced over time to seven brackets per marriage filing status in 2013.
It's debatable how much these figures have on the prevalence of high wealth figures starting in the '80s, especially when we consider the expansion of the middle and upper classes and things like corporate tax policy, but they can at the very least give a frame of reference on the subject. Over the years, these numbers would vary, but the overall ending is that those earning well over the national average paid much higher taxes and were more bracketed in the '30s and '40s even up the '70s before the bar for highest earners started getting lower and lower. These means that for whatever purposes, the government views individuals earning above $400k per year as largely the same taxable group, which can affect regulations, or corporate or capital gains policy
https://www.scribd.com/document/62544151/PL-99-514-Tax-Reform-Act-of-1986
Another factor is financialization of the US economy, the easiest definition of which is the rising portion of national income produced by returns on financial instruments, including good ole fashioned stocks and bonds but especially the design, buying and selling of much more exotic instruments.
A good example is the mortgage-backed securities which boomed in value until the '08 financial crisis.
The portion of US GDP from the financial sector has increased by a factor of 4 since 1945, and particularly since 1980 (Philippon 2008).
The general consensus is this kind of financial engineering is so complex that only a relatively small group of insiders know how to manage it, and tend to benefit disproportionately from it.
Put in a different way, there are fewer people making more money off complex financial schemes than eg pre-1980 when more people made their money off manufacturing and other services.
Check out Grippner 2005 "Financialization and the World Economy" from Univ of Michigan Press.