Why was Dave Pelzer the only child abused by his mother?

Mothers with Borderline Personality Disorder will often take out all their self hate on one child while putting any love they have for themselves onto another child this is a part of their mental illness a common general symptom of which is vilifying or idolising people.

When David left home another brother Richard took his place as the vilified child she put all her hate onto. (Richard has written his own two books about the abuse he suffered which was slightly less severe than Dave, as she was afraid to starve him and cause his death, but otherwise severely abused him after David was removed from her care.

David said that his mother at one point explained she chose him for her hatred because he had an annoying high pitched voice compared to his other brothers.