Piggy could be a very good leader because he is thoughtful, intelligent, and fair. He recognizes that the conch can be used to establish order and organization. It also allows anyone to be heard in turn. Using the conch to gather the boys together is the main tool of establishing unity. Piggy recognizes this and that staying together is going to be important for their survival. When Jack and his hunters split off to form another group, this unity is broken, and shortly after, so is the unifying and ordering power of the conch. Piggy's glasses are used to start the fire. Even though it was not his idea to use them to start the fire, his glasses become an indirect symbol of power and possible salvation (if a ship were to see the smoke from their fire).
Despite all of Piggy's leadership qualities, he is not respected by the other boys. If this had been an island of adults, things might be different. But Piggy is awkward, overweight, and has asthma. The boys associate leadership with the more physically able leaders like Jack and Ralph. Piggy is also known as "Piggy" from the start. This is hardly a way to become established as a leader who commands respect from others.
Piggy and Ralph might be the most responsible boys on the island, but Ralph understands how to lead boys of this age better than Piggy does. Piggy doesn't really consider their age and this is why the other boys tend to see him as a scolding adult figure rather than someone just trying to help. When Ralph or Jack criticize the boys, they tend to listen. When Piggy criticizes, the boys acknowledge him but still view him as inadequate. Even after making the following observations, both Ralph and Jack criticize Piggy for not doing enough or for talking too much:
Then when you get here you build a bonﬁre that isn’t no use. Now you been and set the whole island on ﬁre. Won’t we look funny if the whole island burns up? Cooked fruit, that’s what we’ll have to eat, and roast pork. And that’s nothing to laugh at! (Chapter 2)