You can't maintain advertising hype for very long, is the problem. With so many films coming out, particularly big ones, if you start advertising too far ahead of your release people will zone out and end up thinking the film's come and gone before it's even released. See also Tron Legacy, a film with an advertising campaign which started a year or so before the film's release. By the time it came out, most people had lost interest or thought it had been released already.