The most obvious answer to this question is because he doesn’t want to see you anymore. And he’s too coward to tell you that face to face, or even send you a text.
It’s a horrible situation for the receiver of this. Because at first, you tell yourself he will probably call you or text you sooner or later. But days go by, and you don’t have any news from him. No invitation to see each other. Nothing. You didn’t see it coming when you were together. He used to call you to set dates, to invite you to his place or to the restaurant. You wonder if you have missed some signs. But it’s not easy to tell you will end up getting no news from him. Yet, the feeling won’t let you sleep.
Sometimes, after weeks, or months, you will get some news from the vanisher. He’s probably bored, or just got out of a relationship, and thinks about you at that moment. It means nothing for him. And yes, you’re a fool if you reply to his message (unless you tell him to go fuck himself). The silent treatment also tells him to go fuck himself.
Some of my ex’s were vanisher like that. But the funny part of this story is that I still see two of them from time to time, just to catch up. We don’t see each other very much. I only see one of them twice a year. We just chat, and laugh, like old friends. He never contacts me. From the first time we met, I was always the one who initiated the contact. But he always replied. It’s just that he doesn’t contact me. It drove me mad, and after two weeks of no contact from him, I sent him a text saying we were over. He got mad when he received this, and told me if I wanted to break up with him, I had to tell him face to face. So we met a cafe near my place, when I told him it was over. And I didn’t hear from him, until the day of my birthday, when he sent me a message to wish me a happy birthday. He wished me a happy new year. I ended up going with him to a restaurant, and we had a long chat. That night, I understood I couldn’t expect anything from him. It was easy because time eased the pain, and I dated other men after him. He also dated other women. We’re just happy to see each other, as friends.
When someone doesn’t bother to call you anymore, don’t insist. Yes, that person doesn’t want to see you anymore, otherwise he would have called you. Don’t call him, don’t go to his place. Spend time with your friends, your family, do the things you love, and date other people. You might find someone who really wants to be with you.