Investigations like this are slow and difficult in the best of circumstances. But when the events occurred so long ago and victims have to consider going through the trauma of recalling them (often multiple times over the course of months or years), it has to be handled very carefully. There is frequently a lack of evidence and the only eyewitness will be the victim.
We can read about a case from the 1980s that could not be pursued because the statute of limitations has expired. But the jury will not be allowed to hear anything like this. And there are many other details and information that will not be allowed at trial. Sometimes a case can seem very strong when presented to the public (and to a grand jury) but once you are in court things are very different. Prosecutors are understandably hesitant to move quickly on cases like this, because it feels terrible to put victims through the trauma of a trial just to watch a defendant celebrate a victory.