why is simon thought to be more sensible than others?

Simon shows wisdom more than anything else. Many of his suggestions are not understood by the other boys. Simon's assertion the beast is inside the boys themselves is a universal truth that that is too complex for the boys to grasp. Simon understands that the real problems on the island stem from their own darkness. This is wisdom that, if the boys understood, could have saved their souls. Simon, as a Christ figure in the novel, attempts to bring good to the meek (littluns/Piggy) and even loves his enemies (Jack).