I will now venture the hypothesis that the traditions of social science have moved away from the principles of natural science as a result of a cultural selection process. This can be explained as follows. Social phenomena are so complex that every rule has exceptions. Whenever somebody proposes a theory of cause and effect in a social system, somebody else will find an exception that falsifies the theory. In other sciences, an apparent inconsistency most often leads to a refinement of the theory rather than rejection (Lakatos 1974). But in social science, rejection has often been easier than refinement. Throughout the years, this selection mechanism has slowly depleted large sectors of the social science tradition of falsifiable theories. What is left is idiographic descriptions, definitions, categorizations, and interpretations.I think this may have some truth to it. Why would rejection of theories be more common in social science than in other sciences? I think this is due to perfectionism. Social scientists deal with an important topic: human society. Having a theory in this domain that is wrong can potentially result in serious negative consequences - euthenasia, slavery, racism, Thatcherism - and so on. So: social scientists are more trigger-happy when it comes to "incorrect" theories than scientists in many other fields.
While this whole idea is an interesting possibility, it could be criticized as being a "just so" story. However, no doubt many theories start out their lives in this state.
Probably other factors also contributed. One famous one is the anti-theoretical approach of historians. Historians at some point acquired the meme that observation was theory-laden - and to make objective observations, you had to clear your mind of existing preconceptions. Franz Boas imported this idea into cultural anthropology. This might not seem to be so terrible an idea - but the consequences turned out pretty badly for both disciplines.
I still think it is important to understand where so many social scientists went so far off the rails with the scientific method - and with Darwinism. If you don't recognize and understand your mistakes, you will be more prone to repeating them.