John W. Campbell, Jr., who became editor of Astounding Stories magazine in 1937 and remained its editor through thirty-four years and two name changes, displayed a number of valuable character traits, but a skeptical mind was not one of them. He was a prominent exemplar of what is now known as truthiness, the art of believing something because you want it to be true. In addition to acting as midwife to the psychological theory/religion/con game known as $cientology, Campbell was almost single-handedly responsible for psychic powers becoming a common trope in science fiction, due to his insistence on having his own beliefs reflected in Astounding's stories. The way he did this was to suggest story ideas to his stable of writers (Isaac Asimov's story "Nightfall" is a notable example of the process). Naturally, the story ideas he suggested reflected his own beliefs in various bits of pseudoscience.
At present we have no way of knowing the genesis of Harl Vincent's story "The Morons", which appeared in the June 1939 issue of Astounding, about a year and a half after Campbell first took over as editor, and about a year after Vincent resumed writing after a three-year hiatus. However, given the prominent role played by telepathy in the story, it is reasonable to suspect that, like "Nightfall", "The Morons" was the result of a story idea suggested to Vincent by Campbell. One clue that the story idea originated with Campbell rather than Vincent is that the Venusian natives in the story are not simply human beings, as was the case with earlier stories by Vincent such as "The War of the Planets" and "Venus Liberated", and as would be the case with later stories such as "Neutral Vessel". Instead, the Venusians are quite clearly a different species, rather apelike in appearance, and thus incapable of crossbreeding with humans (as Grayson points out in the story).
The planet Venus we see in the story is the standard version of Venus common to science fiction stories in the period before 1962: a wet, swampy, primitive planet populated by Mesozoic-era monsters and primitive savages. This view of the planet dated back to the 19th century, when the French mathematician Pierre Simon Laplace suggested that the Solar System began as a cloud of dust and gas which gradually contracted, throwing off a series of rings as a result of centrifugal force which coalesced to form the planets.
One of the implications of Laplace's suggestion was that the closer to the sun a planet was, the younger it was. Percival Lowell seized on this idea to suggest that Mars, being older than Earth, was suffering the loss of its original water supply, and that the Martian canals were the result of an engineering project by a native Martian race to bring meltwater from the polar ice caps to the rest of the planet. By the same reasoning, Venus ought to be younger than Earth, and thus ought to be in an earlier stage of its existence, one featuring either cavemen, or dinosaurs, or both. Since Venus' perpetual cloud cover prevented scientists from finding out what conditions were actually like there, writers such as Vincent could be (and were) as creative as they liked.
Mind you, Laplace's nebular hypothesis (as it was known) had fallen out of favor by the time the science fiction magazines appeared, replaced by the near-collision hypothesis of Thomas Chamberlin, which held that the material that formed the planets had been drawn out of the sun by a near-collision with another star. Since the near-collision hypothesis implied that all the planets were the same age, the logic behind the primitive Venus trope was no longer valid, but inertia ensured that the trope continued to be used anyway. The primitive, swampy Venus was finally finished off for good in 1962, when the Mariner II probe discovered that Venus was literally hotter than an oven.