Why does Jack want to get rid of the conch?

The conch represented the democratic society and rules which the boys had tried to establish. Jack wasn't interested in democracy or anyone else's wishes. Jack simply wanted total power for himself alone. It might be more apt to ask, why didn't Jack smash the conch? Of course if he had done that it would have antagonised the other boys, who might have turned against him. Instead Jack simply weaken its power by continually redefining the limits of its authority and gradually attracting more boys into his own tribe and away from the influence of the conch.