I addressed this in another thread. It makes no sense for the rulers of Ferelden or Orlais to be there. If their entire purpose is to de-legitimize the Inquisition with this inquest/trial/thing, then why give the Inquisitor the the privilege of being viewed as an equal by deigning to be a part of the proceedings? Let their underlings deal with it and thus let the Inquisitor know they view Inky as just another underling that needs to be taken to task. It's a power play, pure and simple.
But why would the Inquisitor entertain this show trial?
The Inquisition has the Divine's directive. If Orlais and Ferelden want the Inquisition disbanded or reabsorbed into the Chantry, then the leaders of those nations should meet with the new Divine and the Inquisitor.
The Inquisition is legitimate, regardless of who wants to admit it or not. It would be more effective if the rulers were present.
I think a better explanation is that it is too dangerous to have them all in one place. Remember what happened last time a Divine called a meeting.