Smith-Rosenberg wrote her essay in 1975, at a time when there were few women's studies at all. At that time, she stated, "virtually no one has written about" (1) this subject. She also stated that to the degree relationships between women were studied, they were studied from a psychosexual perspective, and she urged that such studies could and should be conducted from a more social and cultural perspective. The psychosexual perspective, based on Freudian principles, tended to focus on the idea that female friendships were a form of latent homosexuality, an idea that was becoming largely discounted as Smith-Rosenberg was writing.
In her article, she examines through primary sources the lives and friendships in 35 families in the 1700s and 1800s. Her findings are too complex to be briefly summarized, but she speculated that in the periods she studied, there was an environment that supported female relationships in a way that the twentieth century has not supported female friendships.
I do not know if you have read the entire article, but you should if you have not. It is a most interesting glimpse into a very different world, and Smith-Rosenberg's conclusions and findings are an important part of women's studies. I have included a link to the article itself.