Piggy dissuades Ralph for two main reasons. First, Ralph is the only other person thinking rationally and like a 'grown-up' as Piggy says. Ralph is interested in keeping the fire going and trying to be rescued above all else whereas Jack is just interested in meat and hunting. Also, Ralph is the only one who seems to care about Piggy - all the others just make fun of him. If Ralph gave up leadership, Piggy would be at the mercy of the others.