Screenwriters Joshua Oppenheimer and Thomas Dean Donnelly offer an explanation on why they decided to create a brand new story for the new Conan the Barbarian movie.
We could have stretched out a single [Howard] short story, but we opted to create a new one: one which spoke to the medium it was being created for. Our intention was to create something that fit snugly within the existing Conan mythology, which was hardly less of a challenge than making one of the original stories work within a traditional three-act structure.
Script re-writer Sean Hood explains what was wrong with the original Conan script.
Conan seemed to embrace and trumpet a higher purpose. I didn’t think that was true to his character. Some characters are compelling precisely because, despite enormous danger and pressure, they heroically refuse to change… I’m always struck by the way illustrations of H.P. Lovecraft’s monsters look so childish and cheesy, while his stories are so nightmarish. Likewise, Robert E. Howard’s writing isn’t fairly represented by a muscle-man in a loincloth with a buxom babe. There is a gravitas, a raw passion and intensity that can get completely lost.