Why are citizens obligated to serve jury duty

Because it the right to a fair trial

With rights, come responsibilities. Jurors owe it to their fellow citizens to perform this service seriously. So justice depends all on the quality of jurors who serve it and the survival of your own right to trial by jury depends on the willingness of all to serve, so therefore being part of the system and making it a difference makes it a big part of having the right to do so