Because this requires a tremendous expenditure of time and money, which the police as prosecutors cannot justify if in their experience and judgment there is insufficient evidence to convince a jury.
They are not amateurs, unless the case is cut and dry it is quite hard to convict somebody of a crime. A good deal of petty criminals who see their day in court walk away free as their guilt could not be proved beyond reasonable doubt. And this is when there is plenty of evidence to submit.
