I think this is because the people who are ready to take on the challenge register for the challenge so the longest distance is available longer because it's a bigger commitment for anyone looking to run "only" one race. When I decided to register for my first 10K/Half Challenge, I had already worked through my training strategies and mental plan for recovering from the 10K for the Half. Likewise, when I decided to tackle Dopey as part of my first marathon, I had already worked through the mental part of why I wanted to attempt the marathon.
Speaking for myself, one of the biggest things I always struggled with during a race was questioning what was I thinking when I registered for this during the first 2-3 miles of the race. No matter how well trained I was, I always struggled with that. Well, I still have to deal with that, only now that I run the challenges I get all of those bad thoughts out of the way during the first race and then I'm all settled in for the longer races that lie ahead.