According to Raimi, he had read the script that Blizzard had put forth and wanted to put together something better, so he enlisted the aid of screenwriter Robert Rodat. Rodat wrote the story, but when the story was pitched to Blizzard, the company wasn't terribly enthusiastic. He and Rodat continued to work on refining the script, only to find out later that Blizzard had completely rejected what they had done and wanted to go in an entirely different direction. You can read more details on the story from Raimi in the full interview.
