Ralph is holding the conch. Having blown the conch, Ralph is automatically linked as an authority figure by most of the boys. There were also other traits that set Ralph apart,
..there was a stillness about Ralph as he sat that marked him out: there was his size and attractive appearance; and yet most obscurely, yet more 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.
Ralph was close enough to the boys’ recollection of adult authority to give him the role of chief.
Ralph was chosen as a chief because he was the one who blew the conch that led them all together in the island. When the boys decided that they should have a chief, Jack was convinced that it should be him as he was already the head of the choir group. As jack is rude and arrogant all the time, his commands were always being followed. So he knew that his boys would vote for him but the rest of the boys votes Ralph as he was the eldest among them. He had stillness and was attractive and that he had the conch with him and as piggy was his friend and he was the most intelligent guy they thought that piggy could also help him to sort out things. So Ralph was elected as the leader of the boys as the conch somehow seemed to give him powers.