I don't know if we can nail it down to any one factor. I think that there were many small factors that have slowly torn away at the values of personal responsibility, accountability, and others.
The context of the 1940s that I see was a Great Depression following the first World War -- was this "personal responsibility" a result of extreme austerity? I think so. But then, for WWII, we saw men conscripted into the polar opposite of "personal responsibility" -- soldiers followed orders; their sole responsibility was up the chain of command. But when they got home, there were high-paying low-skill jobs available, and blue-collar families could afford to have 5 kids, and a house big enough to keep them, on a single worker's income. Today, it's hard to find that kind of income without extensive skill and experience -- especially when we're factoring in housing costs. Folks in the 1940s didn't need to be nearly so "responsible" as they do today, just to make it with a single kid.