They are not obliged to tell, and there is no way in which the court can know, what their opinion was.

So a man tried on one side of the court-house may be held guilty, and another man tried on the other side of the court-house may be held innocent for precisely the same act.
The other reason is that the court must always decide what evidence shall be admitted.

So if the jury are to be the judges of the law, one authority must determine what evidence they shall consider, and another determine what law shall be applied to it.

For instance, suppose a defendant charged with homicide offers to prove certain facts which as he claims justify the killing.

The Judge says these facts do not, under the law, justify the killing and excludes the evidence.