The most important reasons the U.S. Constitution guarantees a right to a jury trial in criminal cases is Jurors are a group of diverse individuals who must find agreement about the facts of the case.
In a criminal case it is the Government prosecution of a person which broke the law, the Judge will follow up the law, while the jurors must come up with one agreement about the facts, this gives the right for the criminal to be judged by unbiased views, finding balance between facts and law in a prosecution.