If you're wondering why Ubisoft made the decision to omit female assassins from the upcoming Assassin's Creed: Unity's four-player cooperative mode, it's simple. And I'll admit, a little predictable. According to an interview with Ubisoft's creative director Alex Amancio at Polygon, the crew ran into the "reality of production" and found that including female character models is just too much work.
"It's double the animations, it's double the voices, all that stuff and double the visual assets," Amancio explained. "Especially because we have customizable assassins. It was really a lot of extra production work." He went on to clarify that in the game's cooperative mode, players can customize their own gear, but they'll see themselves as protagonist Arno no matter the outfitting. Friends would simply be displayed as other assassins with different faces. With this in mind, since as Amancio noted, they couldn't "cut their main character," the only option left was to give female avatars the axe.
Polygon spoke with level designer Bruno St. Andre about the topic at length as well, who estimated that the team would have had to recreate some 8,000 animations on a completely different skeleton for a female character, but he later noted that it's "dear to the team," so we can expect that it will happen "eventually."