By the time WWII ended Americans had lived through about a decade and a half of very difficult times, including the great depression and WWII. During that time, large numbers of Americans had struggled financially, struggled to find work, or had their life turned upside-down by the war, and as a result they had often had difficulty living the kind of lives they wanted.

After the war, it was different: the depression did not return, employment was high, the economy was booming. Many people had been able to build up a considerable amount of capital during the war (through war work, buying war bonds, etc). After the war, the government subsidized education and low-interest mortgages for servicemen through the GI Bill. So basically the immediate post-war period was this rare combination of pent-up demand to live the "good life" as Americans understood it (marriage, house, kids, car, etc), combined with economic prosperity and a government that saw suburbia, car culture, and a consumer-driven economy as the best way to avoid another depression. The result was that huge numbers of Americans who had postponed marriage and family, seen their parents unable to support their own families or live the life they wanted during the depression, and witnessed all of the dislocation/strife that the war caused, were incredibly eager and happy to start settling into a life of (idealized) domestic bliss.

The baby boom is the single biggest and most obvious product of this embrace of domesticity, but you can also see it through the explosive growth of suburbia, the development of the inter-state highway system, and culturally as well. Throughout the late 40s and 50s, an unusually strong emphasis on marriage and the nuclear family as the way to happiness permeated pop culture: TV shows like Leave it to Beaver were supposed to reflect the stereotype of what Americans felt was an ideal life. In advice literature, American women were constantly admonished to find happiness through marriage and family rather than through education or a career, and families were encouraged to practice "togetherness" - to form close family bonds and relationships with each other.

With this emphasis on leading a "normal" life - which many Americans understood as "natural" as well at the time - other ways of living and other paths to happiness came under suspicion; independent women, gays, and other outsiders were often portrayed as threatening in 50s culture. And in the context of the Cold War, there was often a bit of a parallel between international events and domestic life: the family was seen not only as the route to happiness, but as a bastion of American strength and resilience against communism. And outsiders (like homosexuals) were often seen as a sinister, subversive force, or even assumed to be communist. Intellectually and politically, the 50s were a decade dominated by the idea of "Consensus:" with the depression over, fascism defeated, and communism "revealed" as more or less pure evil, Americans on the left moved to towards the center, while people on the right increasingly believed that a large, prosperous middle-class was the best way to a prosperous and happy society. Intellectuals increasingly shared the notion that the "American" way of life (consumerism, capitalism, American democracy) was pretty much the best thing since sliced bread. Everyone shared the assumption that the US needed to fight communism. I'm generalizing here obviously, but what I'm trying to describe here is the ideal of the "post-war liberal consensus" which is likely contributing to your view of the 50s as a "wholesome" decade.

In many senses, of course, the picture that I've painted of the 50s here is a massive over-simplification: huge numbers of immigrant, working class, black, and Mexican-Americans could not afford to or did not want to live like the families that dominated 50s-era TV. There were a lot of homosexuals who tried to live life as happily as they could, and there were huge numbers of women who tried to juggle both work and a family life, or who chose their career over family. Many of the people who tried to build the perfect suburban family life like they saw on TV ended up miserable or divorced. And there were still radical left-wing and far-right conservative groups around, quietly biding their time and building support while they waited for broader political rifts to appear, for the "consensus" to start to show cracks. And it was not long at all before the civil rights movement showed just how shallow that consensus could be. So it's important not to over-state just how "wholesome" the 50s was. But you're correct to notice that this was certainly the ideal.

Sources:

Elaine Tyler May, Homeward Bound

Lizabeth Cohen, A Consumer's Republic

Joanne Meyerowitz, ed., Not June Cleaver

I am sorry, I am not a native English speaker, so can you explain what you mean by "wholesome"?

If you are reffering to the 50s seen as a happier, less complicated time, it is mostly a myth, but has some basis in the difference between the eras that came before and after.

The 50s were mostly marked by prosperity - there was a boom after ww2 that benefitted the US, keeping employment high and real wages increasing. The labour strife and unemployement that had marked much of the 30s was gone with what was mostly full employement. The counter-cultures, such as hippes and not yet risen to prominence, the Vietnam war had not yet started.

However, to describe the time as one without strife or conflicts is in error. The Korean war, the slowly rising civil rights movement, segregation and the conflicts around ending it and so on. Rock'n'roll and pop also challenged traditional music and values.

There's a tendency to view the past with rose-tinted glasses. We see the things that were built well and lasted from that era, but all things that were crap and are long since gone no-one remembers. Compared to the previous era (depression 30s, world war 40s) and later era (Vietnam war and counter-culture) the 50s might seem peaceful and full of prosperity, but under careful study, they are about as good or bad as one can expect.