The way Conan adaptations were treated was an outgrowth of the way that pulp writers following Howard thought about the material and the way they treated it. For a long time, the people who owned and managed the Conan the Barbarian intellectual property had no notion of Robert E. Howard's original stories being canonical, and this had a direct effect on how film adaptations of the Conan material were structured.
Fantasy, science fiction, and adventure fiction writing existed in a very different cultural milieu in the 1920s through the 1950s. Readers, most editors, and many authors treated the content of the pulp magazines as ephemera, intrinsically inferior literature that would shortly be forgotten. (The very name "pulp" is indicative of this. The pulp magazines were printed on cheap paper made from the least processed wood pulp. The paper yellowed quickly and tore easily, making the original editions quite fragile.) In most cases, this was correct; most of the pulp fiction was good for a quick thrill and was then switfly forgotten. (Note that the pulps were not so unique in this was though. Sturgeon's Law, that 90% of all media output is crap, applies pretty broadly. Part of Theodore Sturgeon's point in pointing this fact out, though, was that it did not just apply to hack science fiction and fantasy.)
Another related phenomenon was that America's pulp SF and fantasy authors of the 1920s and 1930s included frequent allusions to one-another's work. (For example, "The Whisperer in Darkness" includes a mention of "Commoriom myth-cycle preserved by the Atlantean high-priest Klarkash-Ton"; say it aloud if you don't get the references.) While H. P. Lovecraft, for example, created the Cthulhu Mythos, he was gratified to see his literary colleagues writing their own Mythos stories as well. Among the pulp authors of the period, there was a general attitude that characters, settings, and motifs could be freely shared; and less talented authors often produced pastiches based on the writings of the masters.
The prevalence of such pastiches was particularly high when the original authors responsible for creating a shared setting were deceased.
The works of Howard (and to a lesser extent, those of Lovecraft) benefitted from the fact that the author(s) died relatively young, in the mid-1930s at the height of the pulp period. Other authors, producing a continuous stream of new Conan material, kept the character and his setting in the public consciousness, contributing to Conan's enduring popularity. Compare Clark Ashton Smith, who was arguably as good a writer as Howard, but who did not die young and whose works are quite a bit more obscure; or Frank Belknap Long, who lived until 1994 but whose fiction is mostly forgotten.
After the death of prominent writers like Howard and Lovecraft, other authors continued to write stories using the same characters and settings. The quality of these pastiches was extremely variable. August Derleth wrote many more Cthulhu Mythos stories after he strong-armed control over Lovecraft's intellectual property away from R. H. Barlow; however, Derleth just did not understand the enormity of the cosmic evil that Lovecraft conceived of and tried to write stories in which there were equally powerful good entities that might protect the Earth from the likes of Cthulhu and Nyarlathotep.
Similarly, curation of the Conan storied fell into the hands of Lin Carter and L. Sprague de Camp. While Carter was a reasonably competent editor and helped maintain interest in a number of pulp authors who might otherwise have been forgotten, he was himself (in most critics' opinions) a mediocre writer. De Camp could be quite a good fantasy writer, when he stuck to his own ideas, but his pastiches of Howard's work were problematic in many ways. Fundamentally, Carter and de Camp did not understand all the aspects of Howard's Hyborian Age. (A glaring example, and one that made it into the 1982 Conan the Barbarian film, was interpreting the Hyrkanians as Mongols. Carter and de Camp evidently did not realize that "Hyrkania" was not just a name Howard had made up; it was a real region in Iran.) The pastiches were published alongside Howard's original work, giving the impression that this professional fan fiction should be considered just as canonical as the true creator's writing.
Carter and de Camp also took many of Howard's unpublished (and sometimes unfinished) manuscripts and rewrote them as Conan stories, regardless of whether or not that made sense. For example, the non-fantastical "The Trail of the Blood-Stained God" was converted into a Conan story, even though the unedited parts of the text have a clearly different feel from a true Conan tale. (To be fair, however, Howard did this kind of thing himself. After "By This Axe I Rule" was rejected, Howard created the character of Conan to be featured in an edited version of the tale, which became "The Phoenix on the Sword.") This goes to show that the people who were most responsible for keeping Conan material in print did not really appreciate the nature of the world Howard had created; nor did then consider Howard's stories more canonical than their own (or those of other authors they worked with, such as Björn Nyberg, who wrote The Return of Conan).
When it came time to make a Conan movie, this was the environment in which it was made. De Camp was credited as a "technical advisor" to Conan the Barbarian, and his attitude about the canonicity of various Conan stories is evident. Elements from many different Conan stories, many of them not written by Howard, were included in the movie. (I recall once watching the film with my brother. We had both seen it before, but he had not read many Conan stories. As the film progressed, I pointed out which vignettes were taken from specific published tales. For example, the way Conan finds his sword was from "The Thing in the Crypt," a Carter/de Camp pastiche.)
With no notion that there was a canonical set of original Howard stories, writers, directors, and producers adapting the character used material penned by other, less skillful Conan authors as source material. Moreover, there was a general attitude that Conan the Barbarian could be used in whatever way an adaptor felt was most appropriate. Film and television versions of the character were just new pastiches; they differed merely in their medium from the stories written down by Carter, de Camp, and others.