They probably couldn't afford her. I'm guessing they had a short production budget in general, and blew most of their acting budget on Cavill. Apart from Cavill everyone else is either a newcomer or a little known actor.Even though I warmed up to her over time, this in the quote still bothers me a bit, she should look a lot more mature than she does.
Like many mentioned, Eva Green is still perfect for the role, oh well...
Not to say anything of the SFX budget, as demonstrated in episode 6 ("Rare Species"). That took me back to the times of Hercules and Xena the Warrior Princess.