A successful prosecution in any of these cases was always going to be difficult, given the huge time span of which the alleged offences took place.
The best witness in the world would find it difficult to stand up to a good defence counsel, if they were being asked about events that took place 30, 40, or 50 years ago. In all of these cases, there hasn't been any other kind of evidence, like DNA for instance, so it relies on witnesses having total recall, and being strong and resolute on the witness stand.
Lets not forget the Stuart Hall is only in prison because he made a last minute confession.