Common law, case law, moves slowly. It basically crowd-sources notions of fairness and justice over time and turns them into rules. Normally this works well. But when the assumptions that informed the common law were faulty, then precedent drags positive change.
We can see this happening in child custody arrangements. The precedents set in the 1970s when the divorce rate rose were informed by Freudian attachment-theory studies in the post-war era on orphans, as they were the most commonly found victims of fractured families. As attachment theory developed, psychologists started studying mothers and young children. It seemed a logical first layer of detail to examine given the expectations that women took care of the children while men worked outside the home.
When the divorce rate rose in the ’70s and courts had to start declaring custody arrangements, the experts recommended primary mother care because they didn’t have data for anything else. From a 1992 “Origins of Attachment Theory” paper in Developmental Psychology:
Although we have made progress in examining mother-child attachment, much work needs to be done with respect to studying attachment in the microsystem of family relationships (Bronfenbrenner, 1979). Despite studies by Belsky, Gilstrap, and Rovine (1984), Lamb (1978), and Parke and Tinsley (1987) that show fathers to be competent, if sometimes less than fully participant attachment figures, we still have much to learn regarding father attachment.
Formal studies of children in broken homes didn’t really start until the ’80s when there were children of divorce to study and a fierce need for relevant data. And the father and child arrangements that the data recommend look little like the modern arrangements formed under the inertia of legal precedent.