There’s some writers out there who push a conspiracy theory that “John W. Campbell murdered pulp fiction”. JWC didn’t kill the fantasy adventure pulps. They were destroyed by something else that happened at the same time JWC took over Astounding: World War Two.
The war took millions of readers away from their pulp collections and sent them overseas to fight the war. When they came home Conan eviscerating a zealot wasn’t exotic and thrilling. It was a reminder of when their buddy’s belly was opened by a German shell or Japanese bayonet.
While at the war they discovered the greatest warrior didn’t have as much of an impact on victory as a radar operator.
Combine that with soldiers dropping from the sky, artificial harbors at Normandy, encryption becoming “Magic,” rockets flying across the North Sea, and all the other inventions that made the GI a small part of “combined arms” and readers weren’t looking for hand-to-hand action. They wanted to know what the next totally unexpected superweapon would be. They were still shocked that a single A-bomb could destroy an entire city. They wanted to know if nuclear war could be avoided.
John W. Campbell’s Astounding (later Analog) magazine offered the answers they wanted, in fiction form. Heinlein, Clarke, Asimov, and many others wrote about how to avert nuclear war and what new technologies might be out there. That’s what readers wanted.
21st-Century readers are leading safe lives without fear of world war or atomic destruction. They’re “Too wonder-stale to wonder at each new miracle.” So many of those readers prefer 1930s pulp to 1940s hard SF. Unsurprising. And not worth demonizing John W. Campbell over.
(And, of course, many pulp readers switched to watching television when it was available, but I’m mostly talking about the science fiction/fantasy pulp magazines here rather than the larger markets of Westerns, romance, etc. pulps.)