Well, I don't know. And while I, like most here I would imagine, wait on the edge of my seat for information from LE, I know that broadcasting information to amateur sleuthers is and must be low on their list of priorities.
As for why the calls have not been released: I can't guess. I will say that there are some very good reasons why such calls don't get released, though.
I don't know that any of this applies to Holly's case, but these things have happened: Occasionally calls reveal details specific to a crime that LE does not want the general public to know. Sometimes, keeping such details private may enable LE to quickly dismiss claims by nuts who say that they've committed the crime. And there are also ways in which keeping your cards close to your chest can help in interrogations with guilty parties.
There have also been cases in which LE would not want to release a 911 call because details of the calls offer a greater public safety risk than benefit.
Sometimes calls may include erroneous information which, were it not for the heat of the moment, would seem slanderous. Anyone reading this must be familiar with the fallibility of eyewitness testimony.
Now, I'm not saying that the calls were not released for any of these reasons. It could have been one of them, or some other reason, or for no reason.
But I don't presume that it's anyone's right to hear the calls, and I can say that there are some good reasons why such calls might not be released.