Because this isn't just a content update;

http://eu.finalfantasyxiv.com/lodest...5d8e92ba0b2ebb
In addition to the update, we are planning to make major updates to the data centres. Therefore, we have will be conducting a 24 hour maintenance (maximum) starting from Dec. 16, 2013. Since the maintenance period is so long, let me take a moment to breakdown what we are planning to do.
24 hour maintenance (maximum) probably means nothing, this is SE and they'll probably take longer, but if the stars align they might finish sooner...