Why weren't the boys in Lord of the Flies rescued?

The boys are rescued in the final chapter of the novel, when a British battleship anchors off the shore of the tropical island after witnessing the entire island on fire, which was caused by Jack and his savages while they were hunting Ralph. At the beginning of the story, the boys' plane is shot out of the sky and crash lands on an uninhabited tropical island. Since the plane flew off its course and the boys landed on an uninhabited island, there is no way of being immediately rescued. However, the boys manage to build a signal fire on the top of the mountain in hopes of attracting a passing ship. Unfortunately, Jack dismisses his savages from their duties of maintaining the signal fire in order to join a hunt, and the signal fire goes out before a ship sails past the island. The boys miss a rare opportunity for rescue because the signal fire went out, and they spend the rest of the novel unsuccessfully attempting to establish a civilized society. In chapter 12, Jack and his savages are hunting Ralph, who happens to stumble onto the beach, where he sees a British naval officer. The British officer's presence is a sort of deus ex machina, and the boys are quickly rescued.

The boys were rescued in the final chapter of the book.  The reason it took so long is, as pointed out by Piggy in the first chapter, there is a war going on.  Piggy says that no one knows where they crash landed and that there was an atom bomb dropped in the country they are from that killed many people.  There is no way for the boys to communicate with the outside world and there are no adults alive to help them in any way.  The boys range in age from about 12 down to 6 or so.  With no information given to the authorities about where the boys' plane crash landed and with the interference of the war going on and taking people's attention away from the missing plane of boys, it takes several months for the boys to finally be rescued by the English authorities.

Why does the novel Lord of the Flies end with the boys' rescue?

Lord of the Flies should not be considered a rescue, or adventure story, but rather a statement on society.  The entire novel revolved around removing individuals from the rules and expectations of civilized society and seeing the effect that their new freedom would have.  Jack spiraled down into savagery, bringing most of the other boys with him. Evidence of his savage tendencies was his embrace of his long hair and dirty clothes.  Instead of being repulsed by blood, he finds it funny and even smears it on his face.  Ralph, however, rejects his long hair and dirty clothes and longs to return to civilization.  The rescue fire is a priority for him and not Jack because Jack enjoys the freedom to act in ways that would be unacceptable in his former civilized society. 

The boys' rescue happens right as Jack's savagery is coming to a peak and he is about to kill Ralph in cold blood.  The reader, at this point, is judging Jack to be the most savage person; he is exhibiting behavior that is completely unacceptable to the reader.  We judge him and feel great relief that Ralph survives and the boys are being brought back to civilization.

However Golding does not end the novel with Ralph's emotional release of all the evil that occurred on the island.  Instead, he adds the following two sentences to conclude the novel:

The officer, surrounded by these noises, was moved and a little embarrassed. He turned away to give them time to pull themselves together; and waited, allowing his eyes to rest on the trim cruiser in the distance.

By ending the novel with the image of the cruiser, we are reminded that the boys are being rescued, but are being brought back to a civilized society that is in the midst of a war just like the one from which they are being rescued. Ending the novel with Ralph being killed would present the idea that savagery overtly wins in the end.  By ending the novel with rescue, the idea presented is that savagery is a subtle force that is ever-present, even in civilized society.

Who saved the boys in Lord of the Flies?

In chapter 12, Jack and his band of hunters chase Ralph through the forest with plans of decapitating him. After forming a cordon, Jack orders his hunters to set fire to the forest in an attempt to smoke Ralph out of his hiding spot. Jack's strategy works, and Ralph is forced to flee from the underbrush, racing through the forest as trees explode into flames. Jack and his hunters spot Ralph and are close to catching him when Ralph stumbles onto the beach and collapses. When Ralph stands up, he is surprised to see a British naval officer dressed in an immaculate white uniform with a cruiser stationed behind him.

Jack and his hunters follow Ralph onto the beach and immediately stop chasing him when they see the British officer. The officer proceeds to ask Ralph if there are any grown-ups on the island and assumes that the boys are playing a friendly game of war. The officer is astonished to learn that two boys were killed and informs Ralph that they saw the smoke coming from the island. The officer then examines the boys' appearance and says,

I should have thought that a pack of British boys—you’re all British, aren’t you?—would have been able to put up a better show than that—I mean—.

Although the officer is disappointed by the boys' appearance and conduct, he cannot begin to grasp the tragedies they have experienced. Ralph then thinks about Piggy's death and is overcome with emotion. He recognizes his and the others' loss of innocence and begin to weep in front of the British officer before they all sail away from the island.

Who rescues the boys in Lord of the Flies?

Ralph clings to the notion that his father, who just so happens to be in the Royal Navy, will find and rescue them.  Early on in the novel, he comforts the younger boys by telling them that the "Queen has a big room full of maps and all the islands in the world are drawn there.  So the Queen's got a picture of this island" (17).  Ralph encourages the building of a signal fire on the mountain to aid in their eventual rescue.  Ironically, it is not Ralph's signal fire, but rather Jack's vicious fire he creates to smoke out Ralph during his 'hunt' for the other boy that catches the attention of a "trim cruiser" (202).  The British ship sees the smoke--because Jack has succeeded in torching pretty much the entire island--and the boys are rescued by a British naval officer.