There are several reasons that the other boys rarely listen to, and frequently abuse, Piggy. First, Piggy lacks what a necessary trait in any successful leader: charisma. He is socially awkward, and he has a hard time fitting in with the other boys his age. The boys have a hard time seeing anyone other than an overweight boy with big "specs". Piggy is also intellectually superior to the other boys. He has the rationale of an adult and is easily able to see the difference between what the boys should be doing and what they shouldn't be doing.
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.