No female characters will be playable in Assassin's Creed: Unity because of how difficult it would have been for Ubisoft to include them in the game. Ubisoft technical director James Therien explained to VideoGamer that adding female characters to Unity would have doubled the work. Therien noted that the addition of female characters had been on the to-do list but was taken off as development came to an end.
Ubisoft and the team that has worked on Unity are both huge so it is a little bit surprising to hear that development costs were the issue when it came to adding women to the game. Still, it was Ubisoft's decision and is not something that the company needs to be criticized for.