Suu reveals that she chose to stay in her cage while she is talking to Kazuhiko. Why would she want to do that?

If nobody on the outside really loved her, why would she want to? Her mother was only concerned with the money she received by "selling" Suu to the Clover project. She never mentions anyone else, and if her own mother doesn't love her, who else would. Kou seemed to care about her at least to a certain degree. That was why she gave her her last wish. Still, she needed someone who cared about her without being preoccupied with her powers.

If even the one who is suppose to love her hurt her so badly, she is probably trying to avoid further pain. People can only take a certain amount of suffering before they respond to it. Suu became a sort of doll who couldn't experience deep emotions, not because it was impossible, but because she didn't want to. She then became so used to being alone that she forgot all about it.

It was safe to stay in the cage. She would have lived for a long time and never have suffered a great deal. If you don't know what happiness is, you aren't to upset about not having it. It wasn't until Oruha's music stirred old feelings inside of her up that she decided to leave.