As it turns out, this delay was because Naughty Dog wants to deliver the best ending they possibly can. As Neil Druckmann told Gamesradar: 

Take all the time you need, Naughty Dog. Heck, push it further back if you have to (please try not to). The last chapter of Nathan Drake’s saga needs the best send off it can get.

We fans will grin and bear it.  

Uncharted 4 delayed in order to meet our high expectations - 50