Because they are so unthinking and stupidly selfish. Also because if he stays in touch with you, it's a mind game on his part. "If I call and they answer, they must still want to be with me." Puleeze!
"I still care about you." If you do, then don't freaking call.
I am so sorry to hear about all of this. I wish I could take a magic wand and just wish away all of the hurt and pain. I know the feeling...and it does hurt.
Stay strong. I'm sending you my prayers and thoughts.