The most basic reason for the pendulum is because there is no agreement about what kind of person we, as parents, would like our children to become. Here, I think, is where we find a basic reason why a pendulum swings back and forth between recommending permissive and then restrictive childbearing practices. There can be no scientific answer to such a value question. They differ in temperament, aptitude, intelligence, rates of physical and mental maturation. Techniques that might be effective for one child, might not be effective for another.
Parents differ from each other.
The same method used by one parent might not be effective if used by another. It is difficult to experiment with children to test the validity of various theories for ethical reasons. The act of observing behavior in such a setting might alter the behavior being observed. To test a theory that a given childbearing practice might have an effect on behavior in later life requires that longitudinal studies be undertaken and that innumerable intervening variables be controlled for. Such research is often impossible to accomplish.