User blog comment:Mangetsu20/Mangetsu Reviews: The Hueco Mundo Arc Part IV/@comment-4992373-20131024014847

Mental Cage.

Others have pointed it out as well but Aizen deliberetly said this to her in order to keep her there, making her feel like she had a 'duty/role' to play. This made her feel resistant to try and escape at all and stay in her room. After all if you were told "You have a power that could stop my plans" you probably would also stick around and be like "Thanks for telling me, douche"