Because let's face it. This suits this RP so well.
My character would volunteer as well. Everybody is pretty much the same, aside from a few special individuals, since they must have some solid tactical experience when they are sent as military commander. So we can take turn every day?
What about a group effort in the start? Where we all plan out the first battle together and see who emerges as a favorite to become the Lord Comman- errr, leader for the next.
It'll probably be easier to decide in the Camp Phase. There you will get an idea of who you'll be facing. You'll want to formulate your strategy around that.
EDIT: What I mean to say is, some of you may feel more/less confident about the role depending on who you'll be facing. If your unit is an integral part of the proposed strategy, I would probably want to be Commander. If you think you have a good idea on how to win the next battle, I would probably want to be Commander.