We ended things mutually so he could focus on his career. He said he really wanted to stay in contact, but the first time I reached out he was cold and dismissive. Now he’s ignoring me, why is he?

Not enough distance, seen from his perspective. When he said he wanted to stay in contact, he either lied, or he meant a rarer contact than you thought. Either way, he wants a bigger distance in between, than you. This means you either exaggerated some of his relational features, or, you didn't see some parts of him that act against relating with other people. Either way, it's not him who is the problem (seen from your perspective). The problem lies within your perception (and that is if it bothers you that he is cold and dismissive). If indeed you are bothered by his behavior against relationship with you, then re-focus on acceptable and unacceptable standards for you, so that you can dissociate your true partner that you need, from this guy.