Simon is the most spiritually intuitive boy on the island. But, at times, he is unable to articulate his wisdom. He is extremely helpful to Ralph, especially in building the shelters and offering advice. And he is the only one brave enough to suggest that they face their fears and go investigate what the "beast in the air" actually is. But Simon is too quiet and introverted to be a leader in these circumstance on the island. He might be the most sensitive of the boys as well. And while this is a good trait to create a keen awareness of danger and conflict, it makes him too reserved to be a strong leader. When Simon finds a secluded spot, we get the sense that he's more interested in the peace of nature than he is in hunting or conflict:
When he was secure in the middle he was in a little cabin screened off from the open space by a few leaves. He squatted down, parted the leaves and looked out into the clearing. Nothing moved but a pair of gaudy butterflies that danced round each other in the hot air. Holding his breath he cocked a critical ear at the sounds of the island. (Chapter 3)
If this were a religious retreat of peace-loving philosophers, Simon might make a good leader. But these boys need a leader who can take charge and assert some authority. From descriptions of Simon's character and behavior, it's hard to imagine him shouting in anger as a tool to command attention and discipline.