Of all the characters, it is Piggy who most often has useful ideas and sees the correct way for the boys to organize themselves. Yet the other boys...

Piggy is overweight, asthmatic, and even when he offers good advice, it sounds preachy coming from him. Ralph and Jack, on the other hand, are lanky, in shape, charismatic, and commanding. They have typical leadership qualities. And they also appeal to the boys' superficial understanding of what a leader should look and sound like. So, Piggy is mistreated because of his appearance. He is physically weak and soft. As a leader, he has the right...

Piggy is overweight, asthmatic, and even when he offers good advice, it sounds preachy coming from him. Ralph and Jack, on the other hand, are lanky, in shape, charismatic, and commanding. They have typical leadership qualities. And they also appeal to the boys' superficial understanding of what a leader should look and sound like. So, Piggy is mistreated because of his appearance. He is physically weak and soft. As a leader, he has the right answers, but throughout his life, he has been picked on. So, his mentality in offering advice usually comes out frustrated and conciliatory rather than in the commanding way that Jack and Ralph speak. 

Ralph leads by fairness and confidence. Jack leads by intimidation and arrogance. Both boys also have the physical and athletic qualities young boys look up to and admire. Piggy has none of these qualities. So, it is for mostly superficial reasons that Piggy is not treated with the same respect. And as a result of being picked on, Piggy addresses the others with more of a sense of pleading than commanding. Even though he is the most intelligent and reasonable boy of the bunch, his physical appearance and personal quirks strike most of the boys as signs of weakness. 

If the other boys were more reasonable, they would listen to Piggy. As they devolve into more savage behavior, they become even less likely to listen to Piggy. Note the transition from organization to chaos as a parallel to Piggy's demise. First, he loses one lens of his glasses, then another. He becomes more weak, less able to see. The other boys "see" him as this weakening character. 

The theme of how quickly civilized people can devolve into violence is illustrated in the progressive way the boys treat Piggy. At first, they at least listen to some of his ideas (the conch), but as the story goes on, he seems weaker and more ineffectual in their eyes. As the boys lose interest in reason and organization, they also lose interest and any respect they may have had in the reasonable, organized boy: Piggy.