It is true that Piggy is the most intellectual and logical of all the characters in William Golding's Lord of the Flies; it is also true that the boys listen to him less than any other character in the novel. Though these are all proper English schoolboys who are accustomed to obeying rules and acting with decorum, they are also just boys who are now on an island where there are no rules and no grownups to scold them if they are unkind.
Though Piggy is a good thinker, he is also a perfect target for ridicule and abuse by his peers. He is fat, has asthma, and wears thick glasses, just to start. On top of that he wants to organize everyone, and it is clear that these boys are too impatient for that--especially Jack. Piggy is simply trying to repeat every boy's name when he is interrupted. “You’re talking too much,” said Jack Merridew. “Shut up, Fatty.”
Clearly Jack feels nothing disdain for Piggy, and by calling him "Fatty" we understand that Jack feels this way because of how Piggy looks. On the other hand, the boys vote for Ralph primarily because he looks like a leader to them. Just as Ralph does nothing to earn a position of leadership, so Piggy has done nothing to earn the boys' disdain.