"The years from the end of World War II into the 1970s were ones of substantial economic growth and broadly shared prosperity. Beginning in the 1970s, economic growth slowed and the income gap widened."
There were substantial policy and economic philosophy changes around this time. The 1970s are the rise of Milton Friedman and the Chicago School of Economics, which led to the Reagan Revolution in the US and Thatcherism in the UK, as well as the Washington Consensus within the IMF & World Bank. Those were the philosophies that drove the privatization and anti-union movements over the following half-century, which decimated the power of labor and removed restrictions on the accumulation of wealth, which directly led to the current state of affairs.
From CBPP, a left-leaning think tank:
"The years from the end of World War II into the 1970s were ones of substantial economic growth and broadly shared prosperity. Beginning in the 1970s, economic growth slowed and the income gap widened."