Because he really doesn't have any deep lingering feelings about the breakup. He has moved on.
Sometimes we must face the truth, you made the decisions relating to your relationship with him, you chose to stay, you chose to give him chance after chance. No one forced you to do this.
I am sorry that you are struggling to get over this relationship, however, getting out and dating may just be what will help you. You cannot control what he is doing, you cannot control his feelings..or lack of them...all you can do is control you, work on you and move forward with your life.
Life is not fair, we all know that, all we can do is make the best of it.
Focus on you, forget about what he is doing.
Take care...Dolly