I have a friend who used to be a foster care caseworker. About 50% of the parents she worked with had some kind of mental disability, sometimes MR but more commonly schizophrenia and/or bipolar disorder, and those kids usually went to relatives, often the same relatives who had been trying to get custody since before birth. The other 50% were - you guessed it - due to substance abuse issues, and those were the kids that usually ended up in "the system", on top of being disabled themselves from prenatal drug abuse and alcoholism.
A woman on another board whose son became profoundly disabled after birth (bacterial meningitis) placed him in a pediatric nursing home for a while. She said (and I've heard the same thing about other similar facilities) that you would think that the place had parents lined up down the street on visiting day? Nope. She said maybe 10% of the kids at the facility had any kind of outside contact, and also said, "Some of the parents dropped off their precious children, and never even called to see how they were doing." The saddest cases were the ones where the parents couldn't visit anyway, because they were in prison for trying to kill the child, which was how the child became disabled in the first place.
However, I wouldn't be so quick to judge. I used to know (she is now deceased) a woman whose son had the rarest kind of autism, the one where the child seems normal until age 2 1/2 and then retreats into themselves, and no known therapies help them to this day. He was living in a state institution in another part of the country, and she didn't visit but his father who lived in the area did, and he would keep her updated on how their son was doing. (She emphasized that their divorce was NOT because of having a disabled child, and said that it probably would have happened sooner if they hadn't.) It was just too painful for him to see him, when he didn't know who she, or anyone else, was.
