Why does the Mariner stop "one of three" in other words that partitcular wedding guest?

I suppose it was difficult for a scraggy old man who smells of fish to stop all three wedding guests. The wedding guest he stopped is described as the "leader" of the group. Perhaps if an impression is made on him, he might carry the old sailor's story of sin and redemption to the others. Hey, he could use it as a wedding speech!