Single mom of a 2 year old little girl here...also ex-wife to an abusive alcoholic and drug user. In many ways, I feel your pain. In the months following the birth of my daughter, I painfully realized that my XAH wasn't at all interested in our child, unless it benefitted him somehow.
I remember once him asking me to bring DD to this bar he hung out at so he could show her to the bartender there. And I, stupidly did what he asked, so he could show off to his drinking buddies. Come to think of it, he bonked that bartender eventually.
Anyhow, I know the anger, disappointment and frustration you feel. I'm good friends with it. I used to imagine, back when I was pregnant, that my H would want to take our child out for walks, bathe her, rock her to sleep. It was very hard to let go of that fantasy and realize that DD only had me, because her father wasn't ever going to step up. These days, I do my very best to provide an awesome childhood for my daughter and I thank HP that her toxic father isn't around
I hope you can find some insight in Codependent No More; it really did help me realize the toxic pattern my life had been following for years. I also highly recommend counselling if you can find some time for it...It's really worth it to take the time out, once a week, or even once every two weeks, to work on yourself and see the path you've been on.
As for the "reason" behind this abandonment on your XAH's part...well, I'd venture to say that, alkie or not, he has a serious character defect and that your daughter is better off with you as a parent.