At the time I started programming the core isometric engine that would become Fallout, the D&D license was unavailable, or so Interplay thought. So I went after the RPG system that I liked, which was GURPS. At the time I was running a weekly GURPS game, and I thought the system was flexible enough to let me make any kind of RPG setting I wanted. Plus, I had already worked with Steve Jackson Games, who'd let me use the GURPS Space license on some freeware I had written in my spare time. They seemed like a good bunch of people to work with.
After Interplay picked up the D&D license, they almost cancelled Fallout right then so as not to be working on multiple products that would compete with one another. But I pointed out that the setting was so different from orcs and goblins that there would likely be no overlap in the target audience. Marketing believed me, and Fallout was saved from termination.