The reason for this is that a person has the right to be tried by a jury of his or her peers. This means that the person should not be tried by a jury comprised of just one demographic group of his or her peers. It is important that this should be done because otherwise there is too much of a chance that prosecutors will exclude certain demographic groups in an attempt to empanel juries that are more likely to convict.
The best-known example of this is the practice of trying to exclude members of one race when the defendant is of that race. Prosecutors have often tried to do this in order to be more certain of winning their cases. If a jury must be made up of a cross-section of the jurisdiction, this sort of chicanery becomes much less feasible and the defendant is more likely to get a fair trial.