Finally an explanation as to why The Last Guardian was delayed:
http://www.gamespot.com/articles/sony-explains-long-wait-for-the-last-guardian-and-/1100-6428391/
As expected, the delay was mostly caused by rewriting a large portion of the codebase to work well on the PS4:
"The project went on so many technical changes. We decided to move the project to PS4 in 2012, but because the PS3 code was so optimised for SPUs on that console lots of code had to be thrown away and rebuilt.
"It was a tough decision to move from PS3 to PS4 because it was running on PS3. But not at the performance [level] we needed … We knew [we had to decide] whether to continue the game on PS3 with a compromised vision, or move to PS4."
And why they didn't announce it earlier:
"We moved the project to PS4, and we could have said that the project was now on PS4, but I didn’t want to do it until the time when we were confident about our [release] window and we can show it running on PS4.
"My thing was, when we re-announced The Last Guardian, we had to have it totally running on PS4 at the right performance and be confident about the release window that we announced. In our mind, we have the tech, vision, and design down. Lots of levels have already been made, so we are confident about 2016, so we [announced] it."
The whole article is worth reading if you're interested in the development of the game, as well as the linked article discussing The Last Guardian's connection to Ico and Shadow of the Colossus.
