Why doesn't the mistress ever apologize?

My boyfriend cheated on me with a coworker. We were together three years and engaged. The other girl new he was taken and I moved states away to be with him. He is my first love so I gave him a second chance. He told me he was sorry and it will never happen again. We have went to counseling. My question is why doesn't the other girl apologize to me? I don't have it in my heart to cheat and How can someone not want to apologize to the one whom was cheated on?