The text describing Piggy's death reads:
"The rock struck Piggy a glancing blow from chin to knee; the conch exploded into a thousand white fragments and ceased to exist. Piggy, saying nothing, with no time for even a grunt, traveled through the air sideways from the rock, turning over as he went. The rock bounded twice and was lost in the forest. Piggy fell forty feet and landed on his back across the square red rock in the sea. His head opened and stuff came out and turned red. Piggy's arms and legs twitched a bit, like a pig's after it has been killed. Then the sea breathed again in a long, slow sigh, the water boiled white and pink over the rock; and when it went, sucking back again, the body of Piggy was gone."
The imagery used in the above account is very specific and detailed. It is, however, also devoid of any emotion and is a sterile, technical, and matter-of fact description. The impersonal nature of the text suggests that the reader should view Piggy's death as something ordinary, as an inescapable occurrence. Piggy had to die.
Piggy does not die a hero, nor is he a metaphoric "sacrificial lamb." The descriptors effectively portray his death as an ordinary event; it is almost as if Piggy is objectified. Just as the conch is an object that "exploded into a thousand fragments," so too is Piggy an object that "fell forty feet and landed on his back across the square red rock in the sea."
There is no expression of emotion, of horror. Furthermore, the text uses animal terminology to describe how Piggy dies. The phrases "... with no time for even a grunt," "his head opened," "stuff came out," "...arms and legs twitched a bit, like a pig's..." are all descriptors equating Piggy's death to that of an animal, more specifically, a pig. There is no mourning for Piggy, no reflection.
A further inference which can be drawn from this prosaic description is that Piggy's death also symbolizes his rational and logical approach to things, free from emotion, and therefore, his death is unvarnished by any feeling: it is an unsentimental demise.
Piggy represented the pragmatic resolution of issues. He proposed an adult perspective on the problems they experienced, he pleaded for control and for rules. Therefore, when he dies, so does all reason; savagery comes into full bloom. It is therefore apt, in this sense, that the conch, the symbol of discipline, ceases simultaneously with Piggy, who was the last one to have held it.
Arguably, Piggy is not truly a hero in the traditional sense of this word; however, he does exhibit heroic virtue in his insistence that the vestiges of civilization be upheld as the others regress to savagery.
From the beginning, Piggy has insisted that there be rules and procedures followed, knowing that his own country has become great and powerful by adherence to form and protocol.
I agree with Ralph. We've got to have rules and obey them. After all, we're not savages. We're English, and the English are best at everything. So we've got to do the right things. (Ch.2)
Further, he asks the other boys, "How can you expect to be rescued if you don't put first things first and act proper?" Clearly, Piggy is the consummate Brit of his time, recalling the history of his great and powerful nation that owned an empire. (It was after the devastation done to the country after World War II that England lost its colonial possessions, etc.)
Always Piggy is against "bad form," as the British call breaking from discipline and rules. He insists upon the rules of meetings, respect for the conch as a symbol of order, and rationality as opposed to brutish behavior. In Chapter Eight, for instance, Piggy reiterates to the increasingly unruly boys that the most important thing on the island is the smoke from the rescue fire. He has "the intellectual daring to suggest moving the fire from the mountain" after Jack and his hunters have stolen it. In fact, he takes delight in his success at having defeated Jack's attempt to foil Ralph and his group; he helps to fetch the wood for the fire. Certainly, Piggy's intelligence and forethought and absolute insistence upon maintaining rules are virtuous and heroic in the sense of exhibiting bravery in the face of the encroaching anarchy of Jack and his followers.
"I don't care what they call me," he said confidentially, "so long as they don't call me what they used to call me in school."
Ralph was faintly interested.
"What was that?"
The fat boy glanced over his shoulder, then leaned toward Ralph.
"They used to call me Piggy!"
Piggy is a vulnerable character for reasons that are out of his control. He has asthma, he's a bit overweight, he wears glasses, is intelligent, and well educated. Golding paints Piggy in the manner of a stereotypical "nerd." The other boys, out of a need to make him feel bad or themselves feel better, are cruel to Piggy. They see his value (his glasses), but they do not appreciate him for it.
The quote helps show his vulnerability, because it shows that Piggy has been kicked around and been made fun of for a lot of his life. That knowledge endears him to the reader. In a way, it makes the reader really want Piggy to win. It's a classic underdog mentality. We want to see the little guy win. Golding goes one step further with Piggy in that the reader knows that Piggy is right. He stands for a world of order, rules, and law. The other boys, Jack and Roger most notably, stand for a world of destruction.
The reason that Piggy is a hero is because he stands firm in his beliefs. He never gives in to what Jack stands for. Ralph wavers. His physical fight with Jack and enjoyment of the pig kill are evidence of that. But Piggy never once falters in his beliefs that the boys are heading down a path that leads to no good. Tragically, Piggy dies for his beliefs.
Some readers might claim that Piggy is not a hero, because he is never able to lead any sort of change in the boys. Because of his looks and his non alpha-male personality, the other boys take advantage of Piggy. He is used more than anything, and in the end he dies for a cause that most of the boys do not want. But that's precisely what makes Piggy a hero. He stands strong in the face of adversity and is willing to give his life for it.
In Lord of the Flies, William Golding paints Piggy as having vulnerabilities as early as the very first chapter. Many things Piggy says not only reveal him as vulnerable but also as not a leader; Piggy is instead clearly a follower despite the fact that he is actually far wiser than even Ralph.
In the very first chapter, one instance in which Piggy's vulnerabilities are both clearly present and clearly an obstruction is when the boys are choosing who will explore their surroundings to decide if they truly are on an island. Ralph chooses himself, Jack, and Simon, but Piggy strongly wants to come too. Jack refuses to allow Piggy due to Piggy's obesity. Piggy's response in his defense clearly portrays him as a helper, not a leader:
All the same ... I was with him when he found the conch. I was with him before anyone else.
The phrase "I was with him" helps portray Piggy as being with the leader rather than being the leader. The reference to finding the conch in Piggy's response is also significant. What's particularly interesting is that it was Piggy who knew and recognized what the conch was, and it was Piggy's idea to try to use the conch to summon the stranded boys. Hence, Golding is portraying Piggy as not a leader, not due to his skills but rather simply because he lacks confidence and doesn't know his own mind.
A great clamor rose among the savages. Piggy shouted again.
“Which is better –to have rules and agree, or to hunt and kill?”
Again the clamor and again – “Zup!”
Ralph shouted against the noise.
“Which is better, law and rescue, or hunting and breaking things up?”
Piggy represents an adult world. The above quote shows how strongly Piggy supports this concept. The author's simple words here help show the reader how clearly black and white Piggy sees the world. It's either rules and laws, or destruction. There is no middle ground in his mind. There is no room for debate. While Piggy represents the adult world, he is not an adult, but he holds the adult world as good and right. He adheres to rules of law and the enforcement of them. He represents education and logic, and throughout the story, Piggy attempts to remind the rest of the group of those influences.
I'm sure a case could be made for Piggy being a hero, but your question says to explain why he is not a hero. I'll go with a very simple reason. In most cases, the hero of any given story wins and doesn't die. The hero might get beat over and over again, but in the end comes out victorious. Piggy struggles against a power greater than his; that power is Jack, his gang, and the savagery that they are devolving into. Piggy does an admirable job in his fight to remind the boys of law, order, and logic, but he still loses. In fact, he loses his life to his cause. Sometimes the death of a hero may spark change in those people that the hero was trying to influence, but that isn't the case with Piggy. His death doesn't cause Jack or Roger to do anything other than continue down their path even further.
Piggy is not a hero, because he is ineffective at leading any kind of change toward his beliefs and values. He doesn't physically save the boys in a literal sense, nor does he figuratively save them from their own behavior.