Impact zone fits roughly a thousand. so they're aiming for a size of 4 times what they normally draw. Which they won't fill. in an arena that can fill 70times that. Why would you go to an arena with such a huge capacity with the goal of not even filling a quarter of that?
Because their trying to make the best out of it, if they give the impression that they can fill 70'000 seats and its going look worse when they fail, then if they go for less of the crowd. With the right marketing, tickets prices etc TNA might think it is realistic to at least draw 20'000 I don't think it is, but that maybe their aim which would still make it a profitable event. Also its an image thing to promote a show in such a big and famous value gives TNA that extra bit of shine.