I am not sure she did it, I have a feeling an accidental drowning happened and she helped cover it up, by making it look like someone took her and killed her. But they didn't find her in time to know the cause of death and without that you can't prove murder.
It never looked good for her, but there never seemed to be quite enough solid evidence. Many many coincidences and suspicious circumstances. For better or worse our justice system requires charges be proven without a reasonable doubt.