What are some of the policy reasons underlying the requirement that before a person can be convicted of a crime, every member of a jury vote for conviction?

As far as I know ,in  the United States, the verdict in criminal cases must be unanimous. If even one juror disagrees there is a hung jury which in turn forces a mistrial. 

The state must prove that the defendant is guilty beyond a reasonable doubt. This ensures that if they all cannot agree that there is reasonable doubt and it will be a mistrial.