I don't know why, Clint.

It's just the unfair nature of the world, I guess.
Have you ever seen that intervention show on A&E? That show is really hard to watch sometimes, because you see all these people in pain, and one person who is just self-destruction at the center, pulling everyone around them into their black hole.
In every show, they say "I love you, but I can't allow you to do this to me any more. If you don't take steps to change, I can't be there for you any more." It isn't that you shouldn't care or that you don't care, but you can't allow someone to pull you down into their black hole.
You can't rescue someone. They have to rescue themselves. That's true of all of us. We have to be willing to rescue ourselves and do the work. Support people can only truely help if first we help ourselves.
Unless your ex is willing to take steps to rescue herself, there isn't anything you can do. From your previous posts about her, I'm not sure she is at that point yet.