The same reason he showed male Jedi being brutally killed. Order 66 was supposed to be brutal. If he'd shown ONLY female jedi being killed, then I could believe he was making a point about gender. I think the two women were more about token diversity in the Jedi Order. After all, it's not like there were prominent female Jedi in the films before the Empire took over. Correct me if I'm wrong, but I don't think we EVER hear a female Jedi speak, except for the uptight-librarian-stereotype lady. And the blue twi'lek with the midriff was total fan-service.