Well, the new storyline is dependent on certain timed events working appropriately. There were times that the beastmen got lost when advancing for Beseiged, times when the Dark Rider didn't appear which made advancing rank impossible. They just want to make sure they don't try to release a bunch of new content that ends up breaking the first day and limits progression through any of it. Not to mention that chocobo raising may have elements that could potentially be exploited, like the way you could drop your signal pearl to get a fresh one and powerlevel your NPC. Everybody wants fresh content fast, but nobody wants broken content.