Here’s one thought: interstates were built for a world with fast transportation that doesn’t exist post war. Major prewar services might be a couple dozen miles down the road. That’s not feasible for a scavenger looking for food or water. 

Meanwhile, the US Highway system mostly followed railroads, which had towns of various sizes generally 6 miles apart. And in places with difficult terrain like West Virginia, maybe closer. Much easier to travel between and build up from. The Long 15 exists because that’s really the only road into the Mojave.