Particularlly because the reason she went comatose last episode was because she sent pilots (specifically her lover among them) to their deaths against their own allied forces. If she was indirectly responsible for another pilot's death, she believed she would totally lose her will to continue.

I wonder how much of this recurring problem with her is the reason she had turned into Billy's drunken sex slave at the beginning of this season. She had believed then that she sent four pilots to their deaths. Though Tieria did come back safely, she didn't know about it.
(Aha, Billy. There is another side plot that needs some more development.)

Since we more or less have proved Saji's unwillingness to fight, I wonder if that empty seat on the bridge will be filled by Marie. Despite the fact that the goal was to give her a "normal life," I don't think her kinder, gentler personality would be particularly averted towards protecting Alleujah from afar with the Ptolemy II's mighty guns.