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But there wasn’t a fantasy genre before the war, actually. There was Campbell’s briefly-lived Unknown Worlds magazine, and things like Weird Tales, but there wasn’t anything to point a stick at and call “fantasy.” The wartime paper shortages killed lots and lots of magazines (including Unknown) and little genres (railroad stories, etc.) but “fantasy” hadn’t forged an identity separate from “science fiction” or “the weird tale” yet.
And the big techno-boom (which mostly killed Westerns, not fantasy) was still a decade in the future, after Sputnik. The end of WWII created much more of a “let’s get back to the good, normal life” than a “forward into the technological future!” mindset.
You also seem to be conflating the end of WWII with Sputnik, when there was more than a decade between the two events – a period many people think is the richest time in SF history.
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The fact that genre fantasy grew out of genre SF is the main reason they were “clumped” together. Certain kinds of stories (planetary romances, for example) were considered “science fiction” in the '30s through the '50s, but started moving towards “fantasy” afterward.
And there really was no “fantasy market” in the '50s, and relatively few bookstores, either. Towards the end of that decade, the SF market collapsed, too. Actually, that was pretty much post-Sputnik as well – SF was still seen as so disreputable that even when technological advance was of paramount importance, SF was “Buck Rogers stuff” that would only steal attention from Real Science.
Note that the periodical that started as The Magazine of Fantasy changed its name immediately (with the second issue) to The Magazine of Fantasy & Science Fiction – SF was the big brother, genre- and sales-wise. There were certainly fantasy stories, but there wasn’t a clearly defined fantasy genre, and there certainly wasn’t a mass, self-defined fantasy audience as we have today.
The current fantasy market is a product of two things – primarily, the immense success of The Lord of the Rings in paperback in the '60s, and, secondarily, the fantasy boom engineered by Lester del Rey in the late '70s, commencing with Terry Brooks’s The Sword of Shannara and Stephen Donaldson’s “The Chronicles of Thomas Covenant.” Talking about a fantasy genre before the mid-60s is futile, and even the next decade is pretty diffuse.
If you want to talk about particular stories, of course, then there are some gems – a lot of good work was done in the fantastic areas in the '40s and '50s.
As far as I know, what actually happened is that, as fantasy works came to be published in greater numbers, stores re-named their “Science Fiction” section as “Science Fiction and Fantasy” (or, sometimes, reversed). But very few have ever had a completely separate “Fantasy” section, and I doubt any had one which was then amalgamated into “SF & Fantasy.”
Also, according to Locus magazine’s annual exhaustive summary (in their February 2003 issue), there were 256 SF novels published in 2002 – the highest number in ten years (and, I think, ever). Fantasy novels, though, were up more – to 333, also a new high – which may cause the impression that SF is less-published than it once was. According to their charts, there were more fantasy novels than SF in the years 1992 to 1994, and again since 1998.