Never said they kept "everything" in, but people really devalue the stuff they actually kept within XV.
Why?
If the rumour from 4chan is true about engine problems, the 2013 Leviathan event would be the last thing to expect (outside of Versus XV in general). Even the heartless war from KH2 which initially had many different enemies including behemoths on-screen had to be cut down to what it is now due to hardware limitations on the PS2.
It's because they showed so many great things in the other trailers, the stuff they kept in seems trivial, especially when the very noticeable stuff is taken out, and I'm not just talking about the 2006-2011 trailer, I'm talking about the 2013 trailer, hell even FFXV had stuff that would have greatly improved the games narrative cut out(ie the Dawn Trailer WTF happened to that trailer)
When I get excited for something, and the final product is entirely different, and in some cases for the worst(see Leviathan, and that condensed story (see no mention of Stella, but she seemed to have more screentime with Noctis, and overall seemed interesting(Her chat with Noctis about Etro, the light, and focuses, and their supposed duel(didn't even need to be a love story), and tbh apart from story condensation I see no reason as to why they replaced her with Luna to begin with, instead of just improving her character, and building upon it), changes that can happen within the years, and about FFXVI they haven't given me any reason to be excited for it, especially after the state that FFXV came in, that's going to be fixed with patches, I honestly can't be excited after that electric bugaloo, they need to give the mainlines a rest for a bit until they actually have a plan I don't want another FFXV to happen where you show me stuff that actually gets me interested in it(the 2011 trailer, I never saw the Stella trailer, but she seemed pretty interesting I wish they simply improved upon what didn't work instead of removing her entirely, and I wish Nomura's concept could have been built upon even if the director was Tabata himself, and it doesn't get much better when he said that none of the story was cut due to time constraints, and the 2013 Trailer because my god that trailer was the best FFXV trailer for me.
So It wasn't hardware limitations then, it was because of their engine, so because they had to get the game out in 2016, and engine problems the Leviathan fight isn't even close to that awesome sauce trailer feels bad man.
Sometimes apart of me wishes that they made a Final Fantasy XV with no ties to Versus XIII if they wanted to do something different, and actually made Final Fantasy Versus XIII using Unreal Engine 4 if Luminous was causing troubles.
In fact you know what, I wish that Final Fantasy Type-Next was made in Luminous with Tabata at the helm instead of them adding Versus XIII in FFXV knowing full well it wouldn't be the Versus people were looking forward to, then let Nomura and his team use Unreal Engine 4 for Final Fantasy Versus XIII, or if they wanted to call it FFXV fine. I think they should have committed to one or the other if your going to make Final Fantasy Versus XIII which FFXV was originally advertised as then commit to it, actually do it, if you are not going to then don't, don't add any Versus elements, don't add Stella, don't add Noctis, Ignis, Gladio, or Prompto, just have it be it's own thing, and a great game in it's own right, not haunted by the ghost of Versus that would loom over it's head if it's not the original project.