What prevents piggy from being ther natural leader in lord of the flies?

Although Piggy wishes to accompany Ralph, when he sets out to climb the mountain to ascertain whether they are on an island or not, Ralph chooses Jack and Simon to go with him. Appart from that most of the exclusion which Piggy experiences is self inflicted. He can't swim so he doesn't go swimming and he is to fat and lazy to go hunting or help building the huts. He is mentally exluded by his attitude, he thinks and talks like an adult which, combinded with his body fat, glasses and Asthma, serves to set him apart from the other boys.