Lol this started really like a feminist thing xD
I agree that she was a Sith but as everyone else said, she also severed the ties and will be too OP if left as a Sith in-game. So just think of it as a matter of balance within the Force.
This actually means the opposite of what you are saying @Ninjah9 , she is quite powerful in the game and is left out for balance reasons.
Besides, though she was a badass character in the Clone Wars animated series, she wasn't the most loved and most notable character. So personally, I am happy that the devs actually took great notice of Ventress and made her into the Rampaging, overpowered, truck-like hitting annoyance she is today lol