One can only serve on a grand jury if one is a registered voter. Moreover, that registered voter can only serve on a grand jury if he or she sticks around long enough to serve on that jury and doesn't try to outsmart justice by giving silly-ass excuses to get off that jury -- like I did. I let the Eric Garners and Michael Browns down.
When jurors receive that summons in the mail, they believe the court commanded them to attend, not the Constitution. Such a reality inverts role that juries were expected to play. Citizens have lost the sense that jury duty is constitution duty, and that "we the people" are responsible for our government.