Have been hesitant posting about Amy actually because I know it's potentially upsetting for many who liked her and have their own problems. I was a fan too (not enough real artists around anymore), and for me it feels a bit more personal because I lost someone close under similar circumstances, she wasn't much older than Amy, tiny stature as well, so all this helps bring up some disturbing memories.
A couple of times, it could've been me in that situation as well.
It happened I believe because she was in denial, and being a celebrity probably helps you stay in denial much longer. Everyday folks have to take the fall... celebs can hire a good lawyer to get them out of the mess.
I know she tried rehab a few times, her last stay was very brief, it seemed she only agreed to rehab under pressure or direct orders. She was supposed to be winning the battle against Class A substances, however I don't think she ever took her problem with alcohol seriously enough. And her relationship with her former husband had classic codependency overtones. All that didn't help.
The bottom line is that nobody can be forced to get well if they don't really think they have a problem, if they are in denial, and denial is unfortunately a common part of this disorder.
If you are feeling very distressed about this there are support lines available, Amy's case for a long time was a reminder for me of the reasons to stay sober.