User blog comment:Master Sima Yi/Assassinews 28-02-'13 — Assassin's Creed IV: Black Flag revealed/@comment-4999424-20130301091838/@comment-2142534-20130301110752

The thing with ACB and ACR is that they had to stick to the same time period because they wanted to extend Ezio's story. Since this game is in a new time period (relatively) they probably started making it because they had good ideas for story/gameplay, otherwise they would go with a different timeframe.