A full appreciation of just what Campbellian style “Hard SF” really is all about is only possible with an understanding of when it happened and what cultural forces drove it into its temorary and artificial dominance. For that, the most concise summary you’re liable to find is in the preface to Gardner Dozois’s Modern Classics of Fantasy:

By the late nineteenth and early twentieth centuries, most respectable literary figures– Dickens, Twain, Kipling, Doyle, Saki, Chesterton, Wells– had written fantasy in one form or another, if only ghost stories or “Gothic” stories, and a few, like Thorne Smith, James Branch Campbell, and Lord Dunsany, had even made something of a specialty of it. But as World War II loomed ever closer over the horizon, fantasy somehow began to fall into disrepute, increasingly being considered as unhip, “anti-modern,” non-progressive, socially irresponsible, even déclassé. By the sterile and unsmiling 1950s, very little fantasy was bring published in any form, and fantasy as a genre, as a separate publishing category, did not exist.

Although hardly welcomed to the bosom of the literary establishment with open arms, a grudging, condescending, partial exception was made for a specialized form of fantasy called science fiction, mostly on the grounds that it was “rationalistic”, “modern”, “progressive”, “predicted the future,” and so forth. In other words, SF had a didactic function, a social teaching function, that partially excused its excursions away from the “normal world” as it was very rigidly defined in the 1950s. While a faint whiff of raffishness remained, science fiction could be justified as a specialized form of children’s or Young Adult literature– as literature with training wheels on it, something that would teach the kids to be familiar with Science and Technology and to think progressively about The Future (much in the fashion of the famous “World of the Future” dioramas at the 1939 World’s Fair) until they were ready to put away childish things and turn to “real literature” about “the real world.” SF, then, although it did somewhat distastefully deal with things that were “not real,” could be tolerated grudgingly as a necessary evil for children and even some weak-minded adults. The idea that this “teaching function” justified the existence of SF was a note sounded over and over again, sometimes rather shrilly, in the genre magazines of the day. The Magazine of Fantasy & Science Fiction, for instance used to run endorsements on the back cover from “real people,” sober respectable adults like Clifton Fadiman and Steve Allen, assuring readers that it was okay, socially respectable (really!), to read SF– a strategy very similar to the current advertising campaign for Kellogg’s Frosted Flakes that shows a succession of shamefaced adults, sometimes masked or in darkened rooms, admitting that they like to eat Frosted Flakes even though it’s a “kid’s cereal.”

Fantasy had no such justification, and even many of those who admitted (usually somewhat defensively) that they read science fiction would often draw the line at admitting that they read fantasy– which not only dealt, by definition, with things that “weren’t real,” but which had no mitigating didactic function.

In other words, Campbellian science fiction is the only form of fantasy that was allowed to really exist when progressivism was the dominant cultural force. It is the Kellogg’s Corn Flakes of science fiction, the edifying and prosocial alternative to Edgar Rice Burroughs, H. P. Lovecraft, A. Merritt, C. L. Moore, and Robert E. Howard. It as billed as an innovation, but it really was no such thing. And it’s no accident that it is anti-heroic and anti-romance: it was engineered from the ground up for the express purpose of conditioning people to become the sort of “Men Without Chests” that C. S. Lewis wrote about in The Abolition of Man.