Why is Ralph elected chief in Lord of the Flies?

I am working from the 1997 Penguin / Riverhead edition of the book.  Your edition of the book may have slightly different page numbers.

The election of Ralph as chief occurs in chapter 1 on pages 19-20.  At this point in the book, the boys are all gathered at the platform near the lagoon.  Ralph is holding the shell, having just blown it to attract all of the scattered boys to the platform.  Once the boys have introduced themselves by name, Jack says, "We've got to decide on being rescued." There is some talking, one boy says he wants to go home, and Ralph replies, "Shut up," and lifts the conch over his head.  He then says, "Seems to me we ought to have a chief to decide things."

Jack says he ought to be chief, and another kid, Roger says, "Let's have a vote." The paragraph that follows describes the voting and the reasons that Ralph was elected chief.  I've italicized the relevant sentences.

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.

What this paragraph shows is that even though Piggy was the most intelligent, and Jack the natural leader, Ralph was chosen because he had blown the conch (and was still holding it).  Ralph's "stillness" "size", and "attractive appearance" are factors as well, but not nearly as important as the conch. The other boys have associated Ralph with the power of the conch.

There is a political point that Golding is trying to make here, and that is that we often pick political leaders not based on their intelligence or their ability to lead, but rather on superficial things like attractiveness and appearance.