Why is it my ex still responds to me, but the other guy doesn't ever want to speak to me again?

Sometimes I email my ex, just about general things.. I know he used to really get pissed off with me, but now he does actually reply if I do email him. Whereas this guy I'd been seeing at uni more recently, when he ditched me for another girl (my ex also did this to me) the guy I met at uni told me never to speak to him again etc, then a few months later he got in contact saying he had been really nasty doing what he did to me.. after that he never replied when I tried to re kindle contact. However my ex still speaks to me, but he has got a new girlfriend. Why is it one responds and the other doesn't?