Piggy has useful ideas and sees the correct way for the boys to organize themselves. Yet the other boys rarely listen to him and frequently abuse...

All you have to know to answer this question is what Piggy looks like and acts like and how kids treat people who are like that.

I would say that two of the things that kids make fun of the most are kids who are fat and/or clumsy and kids who are smart and don't know how to act "normal."  Piggy fits both of those categories.

We know Piggy is fat and we know he has asthma and needs glasses.  So he is likely to be somewhat unpopular based on those things.  At the same time, he does not have many social graces -- he doesn't know how to make people feel at ease with him.

So that is why he gets bullied -- he is physically unattractive and he does not come across well.