Piggy knows that anyone other than Ralph acting as chief will spell disaster for him. The other boys pick on Piggy mercilessly and Ralph listens to him as a voice of reason.
Ralph is one of the only other boys strong enough to stand up to Jack in order to keep the peace, and keep working toward finding a way off the island and back to their homes. If Ralph stepped away from this, Piggy knows the hope of rescue will be much diminished.
Piggy dissuades Ralph for two main reasons. First of all, Piggy and Ralph need each other - they are, along with Simon, are the only ones who look at their situation from a mature perspective. The other boys are only interested in playing and hunting whereas Ralph and Piggy want to do everything they can to be rescued.
Also, Ralph is the only one who accepts Piggy. If Ralph stepped down from his position, Piggy would be at the mercy of all the other boys and would have no protection whatsoever.