More awareness for children affected by parental drug addiction

My daughter is ten and her dad is a drug addict. He no longer lives with us and has never been in contact since I asked him to leave 5 months ago. This has had an emotional effect on our daughter and it is so difficult to explain to a ten year old that her Dad cannot be around until he sorts himself out. Naturally I don't want to frighten her but how do I tell her that her Dad is addicted to a drug that doesn't allow him to think straight. I am so at a loss as to how to explain things to her because even I struggle understanding and getting my head around why people choose drugs over their children and families. I had given him so many chances and it is awful to say but it is easier explaining death to a child than trying to explain the implications of heroin addiction. Please, how do you explain this to a child without hurting their feelings even more? From Anonymous Mum.