Simon is trying to tell the boys something about the beast, but they don't understand him. Why don't they understand him?

This depends where in the story you mean. Initially he tries to tell the boys that the beast is just them. None of the other boys understand Simon's existential wisdom. In chapter 9, Simon is trying to tell the boys that their beast is actually the dead pilot on the mountain. The boys, however, are in a frenzy and (thinking he is the beast) are stabbing him to death under a stormy sky.