Or another famous young lady...
And, in those famed Baltimore blocks with the white scrubbed steps, the families were mostly working-class, with the fathers going out to work in factories or for the B&O Railroad, and the moms staying at home with the kids. They didn't have a lot, but they had the necessities, and they took care of what they had.
As for what changed, the rising tide of affluence in the post-WW2 years meant a lot of the children of those parents were able to go to college. Many of those kids, myself included, were the first college graduates in their families of origin. During that period, car culture also rose, as Eisenhower initiated the building of the interstate road systems. Not long after this picture was taken, the beatnick and Red Diaper communist underground was active; and by twenty years after that photo, the beginnings of the Sex, Drugs and Rock'n'Roll "youth rebellion" were stirring. Kids didn't want to stay home and work in the factory. Girls didn't want to grow up to be "just a housewife," because the second-wave feminists were already haranguing them that it was a waste of their brilliance, and an oppression by patriarchal, chauvinist males.
AND lawsuits in the early 60s made their way to the Supreme Court to legalize the distribution of artificial birth control to the unmarried; a series of other SCOTUS decisions destroyed the social supports for marriage, for saving sex for marriage, and for fathers taking responsibility for supporting their offspring, both white fathers as well as black fathers. Women's lib, you know. SCOTUS also passed the rulings taking prayer from schools and allowing profanity and obscenity to fluorish. Free speech, you know.
That's just the main points of what changed.
When I look at that beautiful woman in the photo in a modest house dress she may have made herself on her Singer sewing machine, she typifies the young adults of those bygone days, when marriage was a rite of passage to adult responsibilty and parenthood was a sacred obligation, and modesty for men as well as women was a source of dignity. Messing up in those arenas was considered disgraceful and shameful to your parents and community.
The only time we see women dressed so gracefully these days is when Melania Trump makes an appearance.