Because I only followed him do to his ordering me to and my own sense of honor. I also had said to him that only one faction - royalty or military - could exist if Celtia was to continue to exist. As we both have seen, this has already happened.
Also, I have stated that, should I receive the position, I would immediately chase after the Barbs, leaving Dun Eideann far behind.
Also, you need a new military commander. Political generals tend not to do so well in combat, and I have military experience.