I wonder if it's a combination of mid-century gritty realism blended with a backlash against the popularity of kids' fantasy books? IIRC, the 1940s/50s was when books for middle-grade kids really started to become a publishing phenomenon, and Narnia was the latest in a number of wildly popular kids' portal fantasies going back to the late 1800s. I wonder if all of that tipped over into a sort of concern-trolling fear of kids taking all of this seriously and genuinely believing in fantasy worlds, kind of the 1950s/60s equivalent of "D&D is turning our kids into Satanists!"

I mean, a lot of the massively popular "literary classic" adult fiction from that general time period was also about disillusionment and having your childhood dreams wrecked by the realities of adult life, so maybe this is the kid equivalent of Updike and so on.