Piggy's complete lack of survival instinct may have hurt him in a leadership role. If you look throughout Piggy and Ralph's conversations and gauge the importance of some of the things Piggy suggests, there would have been a ton of wasted time on the island. The only thing that would have been certain if Piggy was in charge would be the lack of violence. Although, as tthakkar pointed out, Piggy would never have been in charge. An outside view of a million schools throughout the world would show you a million playgrounds where kids with Piggy's characteristics don't fit in and aren't taken seriously.