Back in June, Ubisoft announced that Assassin’s Creed: Unity will offer a break from the traditional multiplayer experience and will instead feature a four-player co-op mode.  Following this announcement, it also said that Assassin’s Creed: Rogue for the last-gen consoles will be single-player only.  Ubisoft has now explained this change, stating that it wanted to offer a new experience with the new games.  Speaking to Joystiq, the developer said:

With Assassin’s Creed Unity, the pillars of the franchise have evolved thanks to the potential of new generation consoles.  With that evolution, we wanted to bring a new type of multiplayer experience, which was asked for by fans and which we wanted to explore for a long time: cooperative gameplay.  Our aim was to have a seamless single-to-multi experience: This is why we are not opposing single player and co-op; we have ancestral missions dealing with Arno’s story and Brotherhood missions dealing with Arno’s duties towards the Brotherhood. 

With regards to Assassin’s Creed: Rogue, Ubisoft said that it “decided to focus on the core Assassin’s Creed experience.”