It's that age-old bugbear 'insufficient evidence'.
What that translates into is - 'I know you did it, and you know I know you did it, but I can't prove that you did it in court, so I am forced to let you off.'
That is one way of putting it, the other is a complaint made by someone with a grudge and plod put in a position where they dont want to make the call.
This is why it is dangerous naming people before they are guilty. Some will always say no smoke without fire, hardly surprising since the public has no idea of what really happened.