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...
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.
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.