Answer: Because he is not willing to let go of his riches, his title and his queen.
Throughout the play, Claudius gives us information that hints at the fact that he is somewhat regretful. He seems concerned with getting into heaven, and he also claims that what he has done is wicked. However, he does not seem to be truly regretful. If he was, he would do anything to rectify the situation. This would mean giving up his title, his riches and his wife, and allowing Hamlet to become king. However, he is too attached to material things, and is not ready to let them go.
