in the 1950s and later, the highest tax bracket was somewhere in the 90%s
So, the top 1% of the top 1% of high-income earners, found a way for their income that would fall into the top tax bracket, to not be income. They turned it into capital gains or something else. To not do so would be crazy, so literally no one let a significant amount of their income fall into the top tax bracket, in the 1950s.