In a post on the PlayStation Blog by Neil Druckmann, the director of the game, he explained that this was due to some last minute polishes needed for the game, which the studio felt wasn’t up to their usual standards. He also stated that they had to choose to either stick with the original release or delay it, and clearly they went with the latter.
“However, it was during the last few weeks, as we were closing out sections of the game, that we realized we simply didn’t have enough time to bring the entire game up to a level of polish we would call Naughty Dog quality. At this point we were faced with two options: compromise parts of the game or get more time. We went with the latter, and this new release date allows us to finish everything to our level of satisfaction while also reducing stress on the team.”
This is a bummer for gamers, but then again, we suppose it’s better for them to release a game that’s been polished rather than release a buggy game just to meet a deadline. After all, the first game was extremely well-received and it would be a shame if its highly-anticipated sequel would have its launch marred by bugs.