I do not know what kind of mental disability this daughter had, but often mental disability are being considered "crazy" and those people do not receive the same support that physically disabled people receive.
I have a family member (not my immediate family, though) who has mental disability and he looks just like everyone else with no disability, so when he does or says something that's different, or cannot do things most people can, people look at him weird and think he's crazy.
For those who criticize the mother (I'm not saying what she did was right by no means), I just want to say it is harder to live with someone with mental disability than you probably think.
You are constantly careful with what you do, what you say, repeat the same things over and over, you are told one thing and do it, and then get denied for doing it by the person who told you to do it (because they forget).
More than often, those people with mental disability (unless it is very bad), they are not accepted by those facilities, and so you take care of them at home,
Or more than often, parents feel guilty putting those mentally disabled children in the institutions and so live with them, get frustrated, get sad, get angry, feel lost..
Hard to explain but I never realized how hard it is to have a mentally disabled person in the family until I had one (he was born healthy and this happened due to accident).
Not to mention this mother was taking care of the daughter by herself, I'd imagine it was even much harder.