I figure someday when the living players are long gone, maybe the files can be opened. It may be closed because no definitive proof exists and no bodies, although they have a good idea what happened. Why more heartache or embarrassment for the living family with an unprovable case? Kind of Pandora's Box.
Your question: "Why more heartache or embarrassment for the living family with an unprovable case?"
If you're saying that the person responsible is a dead relative of one of the girls, then my answer is this: Because that isn't about one family. There were three girls and three families. All the families deserve to know that truth. As far as heartache and embarrassment goes... I'm sure everyone's heart is already aching. And preventing embarrassment? Sorry. That shouldn't even be a consideration.