Also, adding female assassins would not be as simple as "create a model". Each added character requires dozens of animations (climbing, falling, running, using each weapon), full voice acting for each of those situations (every time "he" is used would need to be re-recorded with "she"), each cutscene would need to be reviewed for timing issues, and the bug testers would need to do everything twice to check for graphical bugs. While they probably could have done it, something else would need to be cut.