Is a part of that because women weren't expected to have jobs in the 50s, though?
Men employment rate is down from 80% to 65%, but women's employment rate is up from 30% to 55% from the 50's to now. In total the average employment rate for everyone has increased over 50 years from 55% to 60% in the US.
And there's no reason to think that we need a larger labor force than we had in the 50's - the opposite in fact.
The problem, I expect, simply comes down to the size of family units (by which I mean "clusters of people willing to internally redistribute income.") The fewer of those you have, the more individuals need to either be employed for themselves, or get income transfers from the government, to be able to eat/live. And I have the sense that there are far more people living on their own than there were in the 50s.
[+] Legit historical reference so should not shut down the discussion...no godwinning here.