I guess they practiced it very often and beat some teams on it in practice so they were really confident on it. And after the Vitality match they thought "Okay, we did some mistakes but hey, we can fix it in the next hours and beat G2 because they suck on mirage" ... but in my opinion they should not play it because they have a really good map pool and are beasts on other maps. Especially Xantares could have fuck all players on the other maps better than on mirage because mirage is a really hard map which needs much good communication. I don't think that they will play mirage again. At least it will be not their pick. And on a major they should not test things ... that was stupid. They should improve their good maps even more.
PS: sorry for english, je suis une baguette.