While Assassin's Creed didn't start out as an annual franchise, the series has shifted to that formula over its lifespan because Ubisoft wants to keep fans happy.
Consistent yearly releases haven't worked out well for many franchises in the past, with a few outliers like Call of Duty and several sports franchises able to thrive on such a cycle. However, in recent years, Assassin's Creed has established itself as a series capable of maintaining a yearly pace. Though some detractors still disapprove of such a cycle, Ubisoft believes its fans are enjoying the frequent content.
Speaking with Eurogamer at GDC, Ubisoft Montreal CEO Yannis Mallat said he doesn't see the fanbase growing tired of Assassin's Creed, and the numbers would tend to back him up. "No. The players will tell us. Right now there are more and more coming into the franchise, so I don't see that day," Mallat said when asked if there was assassin over-saturation. "It's our breakthrough. When you have quality content, the frequency of coming out with the game is not an issue at all."
Considering Assassin's Creed 3 sold 12 million copies, it's hard to argue with Mallat's reasoning. "On the contrary, people expect more and more of that content," Mallat continued. "So it's natural to be able to provide that content. The gamers are happy, and it's our job to make them happy."
Assassin's Creed 4: Black Flag is due out later this year, which will be the sixth console game in seven years, and eighth in as many years if you include handheld games. Clearly more and more people are turning out for the franchise every year, so until there are some diminishing returns, it looks like yearly Assassin's Creed are here to stay.