That's also the time period that immigration to the US was at its lowest. The Immigration Act of 1924 strictly limited the number of immigrants allowed. That law was reversed by the Immigration and Nationality Act of 1965. Only 5% of the population were immigrants by the 1970 census, the lowest in US history. It's close to 15% now, a level which hasn't been reached since around 1900.
Income inequality plummeted because immigrant labor wasn't allowed to enter the US to drive down the wages of US workers.
H1B labor drives down the wages of US workers. Illegal immigrant labor drives down the wages of US workers. If you care about income inequality, then maybe consider supporting enforcing the immigration laws, and maybe consider supporting ending the H1B and other programs that drive down the wages of US workers and increase income inequality.