I'd just explain to her that something happened that made it so you couldn't go, and that you were very sorry. No need to blame anyone or anything- She will learn to understand and see the patterns of what her dad is doing once she is older, but now she's too young for that.
Just make sure she knows you wanted to be there, you couldn't be, that you love her very much and that you were sorry you didn't make it.
I'd also maybe speak with the teacher and let her know that you don't always get memos that were sent home so maybe ask her to notify you as well
Hope that helps :)