Piggy has been the object of derision since the other boys learned that his nickname was "Piggy" at the beginning of Lord of the Flies. The other boys consider his physical limitations--his girth, bad eyesight and asthma--as weaknesses that they find humorous. They also find his overserious personality and intellect boring, and it gives the boys yet another reason to deride Piggy whenever possible. In Chapter 9, the boys choose Piggy as their target once more. When he and Ralph arrive at Jack's party, one of the boys takes a piece of hot, roasted pig and burns Piggy. Piggy screams and begins dancing, much to the delight of the others.
"Piggy once more was the center of social derision so that everyone felt cheerful and normal."
Just think about every school you've ever been in. Who gets made fun of? Generally, it's going to be some fat kid who is smarter than everyone else. Piggy is also really socially awkward -- after all, he's dumb enough to tell people that his name is "Piggy."
So here you have a fat kid who's smart and socially inept (and wears glasses and has asthma). I don't think it's surprising that he gets made fun of. Sadly, it seems to be the way of the world.
Piggy is the conventional target. The derision aimed at him is a carry-over of the lives the boys had always lived. Picking on the fat kid is a comfort for the boys on the island because it both connects them to one another and connects them to the world they have been separated from.
Piggy was a easy target for the older boys, well in their minds away. His nickname was Piggy he had eye problems and could not see without his glasses. Piggy had health problems his breathing, his overall smartness . He was the kid that was a little over weight had glasses and was smarter then most of the others that were there, so because of that they made him their scapegoat their target.
Piggy is the odd one out. The boys make fun of him because he is different. He is heavier than the other boys. He shows more vulnerability.