Amid the Age of Despair, the fall of the pulps, and the Futurian revolt is evidence of a boom in science fiction publishing.
One genre that is actually increasing in strength [in 1951] is Science Fiction. In 1946 it occupied only 3% of the market, but every year since then has seen a rise in its percentage, and in 1951 it reaches its highest level yet. In part this was due to the perception– still regrettably widespread– that the pulps were somehow the province of science fiction– that they belonged there rather in the slicks, so that those interested in writing science fiction perforce had to write for the pulps. This, despite the historically low performance of the genre in the medium and despite the presence of Avon Science Fiction Reader, Galaxy, Imagination, The Magazine of Fantasy and Science Fiction, Other Worlds, and other science fiction digests. In part it was due to the success of two major science fiction anthologies, The Best of Science Fiction and Adventures in Time and Space, both 1946, which spurred publisher interest in science fiction.
Nevins, Jess. The Pulps: A Yearly Guide (pp. 173-174). . Kindle Edition.
So how does this observation fit with previous claims of readership stagnancy?
First, the metric used here is number of titles published and percent of the pulp catalog occupied by the genre, not sales. The 1950s was a time of flux for the short fiction market. Several pulp companies, most notably Street & Smith, cashed out of the pulps entirely, contracting the number of titles published. What few titles survived were transitioning to the digest format, which also removed them from this pulp metric
Second, this boom in titles was driven by publishers, not the market. Science fiction histories have mentioned that the science fiction readership for pulps and digests combined through this period was estimated at a flat half-million readers (a claim I am continuing to source). Perhaps the clearest evidence that this boom did not lead to increased sales is that this is the time when first wave of Campbellian writers--to include Isaac Asimov himself--were departing for more lucrative genres. The increase in titles did not translate into an increase in writers' pay. This would continue well into the Golden Age, when editors noticed that the same handful of names were on the covers of everyone's magazine.
This would not be the first time publishers chased a trend only to be disappointed. Nor was it the first or the last time publishers produced what they thought people should read as opposed to what readers wanted, whether it was Hugo Gernsback's attempts to channel an interest in science into an interest in his science magazines, John Campbell's amputation of science fiction from its Gothic roots, or Patrick Nielsen-Hayden's (alleged) assertion that there are too many libertarians in science fiction. (Another common assertion needing sourcing, although there's enough hearsay to support it.)
But what of the fiction that inspired this transparent attempt to capitalize on trends? Adventures in Time and Space is a book of the Campbelline Revolution, with only a handful of works that predate Campbell's editorship. The Best of Science Fiction shares a heavy leavening of Campelline works, but reaches back into the pulp era, the dime-novel era, past Arthur Conan Doyle and H. G. Wells to Edgar Allan Poe. Combined, the two create a picture of the canon of the Campbelline branch of science fiction. Absent from both anthologies are works from Weird Tales, Amazing, or the general adventure pulps that were published after Astounding's founding.
It is on the prestige of these two anthologies that Campbell's strange and (internationally shunned) branch of science fiction became the dominant one of American science fiction, despite Weird Tales selling just as well and Amazing setting circulation records yet to be matched. And publishers tried to chase the prestige. However, it is clear that the increased supply did not translate into increased demand.