Vast land was easy to steal when 9/10 natives died from disease. The others lacked equivalent technology and weaponry.
Slavery ensured the newly established federal government made good on its debts. A good credit score means a SHITTON for the success of any state.
Abundant (stolen) land with loads of natural resources settled at the time industrialization ramped up. Lucky.
Literal boatloads of immigrants from all over the world prop up the population, increase production, and suppress wages. Great for industrial growth (like china recently).
And that's all before the 20th century.
That said:
one factor is that computers started really becoming available to businesses, so you really had an automation boom.
one factor is the cold war ended, which changed... a lot of things, including capital flows. That capital shuffling may have helped prosperity.
one factor is the boomers hit their financially most productive years.
Further, housing for GIs in the 40s and 50s was subsidized, thus changing the entire housing equation, and, further, women had not entered the workforce in large numbers, and, further, we require many more safety features on cars.
These factors all are major portions of the story when comparing against your prototypical factory worker story, and mean that a simple narrative of Unions => Better Life doesn't work. Although unions do need to become much stronger in American life, they won't unspool us back to the prior era.
and, tacking a paragraph on in my edit: probably want to talk to some black people about how great those years were. It's entirely plausible that some of the White American prosperity was due to heavily underpaying the Black Americans and treating them very poorly.