I wager this is due to a company edict issued a little while back; they supposedly won't be very forthcoming with showing off any particular games or announce anything until it is at a certain point in development/space of time before it is released. I wonder if what occurred last generation (the premature announcements of the whole of Fabula Nova Crystallis, FF Versus XIII, Crystal Chronicles: The Crystal Bearers and others before they were even properly in development, or a sufficiently tangible state of development) is what led them to this new policy. That's all well and good - only a moron would put the cart before the horse, and a well-managed process of game announcement up to release is what everyone should do - but it's so conspicuously ill-timed at the moment when we are still waiting to SEE (i.e. it is all well and good dropping hints of gameplay, but how about SHOWING us?) a game announced over eight years ago.
Of course, this would make Kingdom Hearts 3 an anomaly. If there's a mandatory new policy, how did that one make it out when it's still so early in development? It was probably an exception for two reasons: fans already suspected that Osaka is working on a mainline KH game, so they may as well let the day out the bag and let fans have something else to officially look forward to; and because they perhaps felt obligated to compliment FFXV at E3 2014 when other major publishers were unveiling their own next-gen projects.
I really do sympathise with everyone currently a wee bit ticked off. Is there a likelihood that it may actually show up somehow in a closed event? Possible, but as said by other posters here, what on earth does one gain from doing such a thing? Isn't it better to be transparent and TELL your fans and customers that you have the next mainline FF game to show instead of treating it like some super exclusive invite only yacht club? I personally think they may actually refrain from showing FFXV in a few weeks precisely because of what I'm suspecting in the above paragraphs. They have other games - it doesn't matter how new, not new or on what platforms they are, as they're still stuff to sell - that will be out sooner, and the PR department seems keener to focus marketing on those games before they think about marketing FFXV, which is by all bets, still a way off yet.
I'm afraid even someone like Nomura promising that there will be FFXV anytime between E3 and the end of this year isn't able to go up against PR edicts.