Have you thought that maybe they harbored a secret hope that by pressing for divorce, their mother would change her ways and stop drinking? And then the both of you would get back together? Having you move on ends that particular fantasy, which means that the grief has come on full force. And since you are the Dad, you still have a role in protecting them from those bad feelings. How do you stop those bad feelings? Stop dating!
You may be onto something because its the daughters that refuse to communicate with their mother that are having the big issue. Whats interesting is that one of them initially was the most open to me dating. But she was still speaking to her mother at that time. Since she quit speaking to mom she has changed her attitude about me entering a new relationship