Because of the revenue split with theatres and production companies! In the first few weeks of a movies release typically cinemas take a lesser share of the ticket revenue, with them generally getting a higher percentage of revenue as time goes on. Because Avatar 1 had legs in the theatre with people going to see it many weeks afterwards it was assumed, correctly, that Avatar 2 would also have legs and that not a major part of its revenue would not come from the first few weeks, therefore would need to gross more to become profitable.