Lack of hard evidence has stopped them from prosecuting him, most likely.
Although gambling is the least serious of his charges, there was more possibility of finding something incriminating there since things involving money usually have a paper trail which can be traced and used to charge people, but seems that hasn't been successful again.
Either way, the most punishment we can hope for at this point is for him to be completely done with his music/celeb career and vanish from the public eye. He'll still live a comfortable life, but it's better than nothing.