Right at the start of the novel, the boys elect Ralph as chief. Golding makes clear that this is a slightly unexpected decision:
Jack started to protest but the clamor changed from the general wish for a chief to an election by acclaim of Ralph himself. None of the boys could have found good reason for this; what intelligence had been shown was traceable to Piggy while the most obvious leader was Jack.
Piggy, even here, even right at the start, is the brains of the operation. He has the "intelligence" visible even at this stage, and he also has the practical, common sense. He's the one taking name so that they know how many boys they are. He's the one who suggests a small signal fire. He's the one who becomes closely associated with the conch, with democracy, with Ralph as elected chief, with civilisation, with assemblies... and so on.
Ralph realises Piggy's qualities in the last page of the novel:
Ralph wept for the end of innocence, the darkness of man’s heart, and the fall through the air of the true, wise friend called Piggy.
But the other boys don't, as you rightly point out, throughout. Why is this? Piggy is fat, wears spectacles, has "ass-mar", is against fun and hunting, and almost irritatingly worthy. He's also a social outcast: we never even find out what his real name is! And because the boys don't think he's cool or fun, his common sense goes largely unnoticed. A life lesson there, Golding clearly thinks.
Hope it helps!
If you look at the boys as different parts of society, Piggy symbolizes wisdom, common sense, and intelligence. Piggy is an outcast because he is fat, has asthma, wears glasses, and lacks social skills. He also has bad grammar and seems to come from a lower-class family. In our society, people like Piggy are often ignored or underestimated. The voice of wisdom is often ignored if it is not presented in the way that people expect. Piggy would be ideal as a leader, if only the other boys would accept him.