Most Helpful Girl
Because, on some level they get addicted to the pain. The abuse is also so gradual that their boundaries stretch just enough to leave room for just a little more abuse every time. They become blinded and brainwashed. They start thinking they don't deserve better. They start thinking that this is their only shot at love. They become too comfortable with the person they're with and start justifying their behavior because they're scared of being alone and too dependent on their abuser.
I'm assuming this is why you didn't leave.