I am a former decathlete, kind of. I agree with Roger. All three are reasons to some degree. A lot of the events are very technique oriented. Also, some people are just better at sprints than long distance. So, they might be able to compete with the dedicated sprinters, but not the long distance runners. Thats the way I was with the hurdles and pole vault. I had always done those and sprinting, so I was strong in those events, but not so strong in the others. (I was TERRIBLE at high jump. I seemed to lose all coordination).
As for the workouts, we didnt do much that looked like Crossfit. We alternated between long and short distance days and worked on different events on different days. Its a rough schedule. I cant recall anything that really resembled crossfit, except for interval runs maybe.