An interesting bit of info I found in
Stone: The Controversies, Excesses, and Exploits of a Radical Filmmaker by James Riordan. Scott was approached to direct Oliver Stone's batshit insane draft of
Conan the Barbarian.
QuoteSince the studio wouldn't accept Stone as director, Pressman approached Ridley Scott. Scott was enthusiastic, and Stone wrote the project with him in mind. Though he liked Stone's first draft, Scott then became interested in doing another film instead.
"Ridley Scott backed out at the last second," Stone elaborates. "I begged him not to. I went down on my knees to him. This was after he made The Duelists. Alien was finished, but we hadn't seen it yet. He said yes, and then he said no. It broke our hearts when he decided to do Blade Runner instead."
Going by the time frame, I'm guessing he backed off Conan for the same reasons he quit Dune, not wanting to deal with the large scale production after his brother's death.