Since they are trying to appeal to a greater fan base, it makes sense to have a woman in the main group. That attracts the female audience to the movie.
Plus from a real life perspective, Zoe is a bigger star than Urban.
And if you think about it, if you saw the same poster with the Doc then you have 4 men all featured. Doesn't look right if you want to attract different people. So a woman makes it work. Plus like I said, its Zoe, so more people are gonna be like "hey its that chick from Avatar...must be a good movie then"