This is from the Congressional Budget Office, hardly a gang of socialist radicals.
It doesn't cover the entire time period of the OP, but it supports the basic point.
In the US, as in most of the world, wealth is power. Economic power, political power, military power. In the end, privilege is enforced from the barrel of a gun. Ask the early unionists who were massacred.
We can nitpick over details of what minimum wage is in what area, etc., but the trend is clear: the rich get richer. The poor get poorer. The bottom 50%, which you can barely see at this scale, gets a smaller slice of the pie, while the top reaches for the stars. Or Mars.