Well, it's all here on page 29:
“Shut up,” said Ralph absently. He lifted the conch. “Seems to me we ought to have a chief to decide things.”
“A chief! A chief!”
“I ought to be chief,” said Jack with simple arrogance, “because I’m chapter chorister and head boy. I can sing C sharp.”
“Well then,” said Jack, “I—”
He hesitated. The dark boy, Roger, stirred at last and spoke up.
“Let’s have a vote.”
“Vote for chief!”
This toy of voting was almost as pleasing as the conch. 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. But there was a stillness about Ralph as he sat that marked him out: there was his size, and attractive appearance; and most obscurely, yet most powerfully, there was the conch. The being that had blown that, had sat waiting for them on the platform with the delicate thing balanced on his knees, was set apart.
“Him with the shell.”
“Let him be chief with the trumpet-thing.”
Ralph raised a hand for silence.
“All right. Who wants Jack for chief?”
With dreary obedience the choir raised their hands.
“Who wants me?”
Every hand outside the choir except Piggy’s was raised immediately.
Then Piggy, too, raised his hand grudgingly into the air.
“I’m chief then.”
Thus, it's not so much that Ralph assumed the role; he was duly elected. And while his size and his looks set him apart, he was seemingly neither the most intelligent nor the most powerful. What made him the obvious choice for chief, for the leader of the boys, was the fact that it had first ocurred to him to blow on the conch. That act, of making a sound in order to bring people together so that everyone could speak to the group and be heard by the group, that knowledge, that ability, that wisdom, that belief in the dual powers and importance of freedom of assembly and freedom of speech, made Ralph the embodiment of a natural-born leader, made him the chief.