Two negative adjectives that come to mind when thinking about the character of Piggy are "weak" and "whiny." He is physically out of shape and struggles to even walk through the creepers and keep up with the other boys, and then he whines about how is asthma is troubling him. He may be smart and practical: for example, his recognizing the usefulness of the conch as a tool to call everyone and later as a symbol of authority; but he doesn't have the physical presense of Ralph or Jack that comes with age and experience. He will not be successful because there are other boys who inspire confidence with their mere presense, and especially at the start of the novel, the little boys are in shock and are going to be drawn to an adult figure. Piggy's behavior and attidude seem childish, especially to the little children. Even Ralph, who does appreciate him, gets annoyed by him as well.
As the novel progresses, both Jack and Ralph solidify their positions as leaders of their prospective groups. There is no chance that Piggy is going to be able unseat either of them -- especially as Jack becomes more fierce and powerful. I would also add that Piggy doesn't truly want to be the leader, so his lack of drive wouldn't inspire anyone to follow him or even take him all the seriously, especially by the end of the novel when he nearly blind and seriously weakened.
Lord of the Flies has characters that represent human traits. Therefore, these characters are not the typically developed characters of fiction books. The character of Piggy in William Golding’s Lord of the Flies is a perfect example of a person who may not be the best looking physically, but despite this, he has the right qualities of an excellent leader. Piggy represents the adult on the island; he represents the logical side of humans in the Lord of the Flies.