How Do You Tell Your Child Why HER Father Is Not In Her Life?

You simply tell her the truth. Start as soon as she asks and keep it age appropriate. Never have anger in your voice and make sure she knows and understands that it's not her fault. When she asks again and a later age, tell her the truth again. She doesn't need to know all of the bits that are none of her business.
I raised my children by myself when their father left us, we never seen or heard from him again. The advice I gave you is the same I used for my children, they didn't grow up angry, and they have no guilty feelings about themselves. I just kept the truth as "matter of fact" as possible because I didn't want to raise angry kids, the anger is mine not theirs.

Chatter is correct. But I should stress that you should keep things neutral. I do not know how you feel about her father but as most people would, I am sure there is a lot of resentment and a little hatred for him. Do not let this come across in any way when you tell her about him. She will make her own decisions about him and if she ever gets to meet him, she will then have a better understanding on what kind of person he is. Stress that it is not her fault at all and that sometimes people get spooked and run instead of taking on the challenges put in front of them. Those challenges can be the best things that  ever happened to us (can you tell I am a father of 3 and they are the best things that ever came into my life).