P.W.
How sad for your little girl.
Do not try to tell her why her situation is not bad. Instead, when she is sad about it, mirror her emotions, telling her that it is sad that she doesn't get to see her father. And don't follow that statement with a "but." (Don't call him her "dad", the word "dad" has to be earned. He's only a father.) Her emotions need to be validated. Everyone needs to have their feelings validated. Also, very important - be sure to stress that it has NOTHING to do with her. Tell her some people are unable to be good parents, or however you want to explain his crappy, selfish, immature, uncaring behavior.
I learned the mirroring technique from a speaker at my kids' school many years ago. Having emotions mirrored is a strong human need. If you tell people why they should NOT feel the way they do, it makes their feelings stronger.