Why do some cases get dismissed?

If the assistant district attorney handling a case determines that there is not sufficient evidence to obtain a conviction, he may file a motion with the judge asking that the case be dismissed. This action is taken only after the case has been completely investigated, and normally after the police have exhausted all avenues for obtaining additional evidence. Even after probable cause has been initially established, a key witness may no longer be available, a defense motion suppressing critical evidence may be granted, or new evidence casting doubt on a defendant’s guilt may come to light.