Because telling them takes the power away from her. If she is successful in blindsiding them with her idol, she draws all the votes to herself and then single-handedly decides who goes home. It’s a massive move that she alone gets credit for. Of course, it hinges on them not finding out, but George was just too savvy for her. She would have pulled it off if she had spent the day pretending to look for idols. She either got lazy, or she is not in the know about George’s spying tendencies.