It is important to note also that the conch is something of the island already. Piggy's glasses came from elsewhere to the island which makes it a better choice for representing knowledge, insight, and civilization.
When Piggy's glasses break, it also represents a literal and figurative "blinding". Not only is Piggy no longer able to see, but the boys themselves have lost sight of the possibility of rescue. They give everything of their former lives up for the new life on the island. This blindness overshadows social order--a sort of "can't see the forest for the trees" existence. They forget all they knew as civilized individuals and become completely immersed in the barbaric lifestyle they have adopted under Jack's leadership.
The breaking of Piggy's glasses indicates a loss of control. The boys are losing control of their humanity and their civilization and becoming more like savages. It is important that Piggy's glasses break in the chapter titled "Painted Faces and Long Hair". These things represent a rejection of social standards, which is further emphasized in the violent killing of the pig. This degeneration towards savagery will lead to anarchy, murder and chaos on this island.
Piggy's glasses, and not the conch, represent civilization and standards of behavior. Glasses are associated with learning and reading, which are pillars of society. Also, on the island, the glasses are needed for the fire, which is the first step towards controlling the environment and not being controlled by it. The conch could not signify this shift towards savagery because it only represents power and control. Even the most wild of animal communities have heirarchies of power. It is the careful and reasoned management of power that leads a community towards the accepted definition of "civilized".