Answer:
Women were always put on the sidelines whenever it came to engaging in public spheres. If we go back to earlier centuries, we'll find that women were considered academically weak and no one took them seriously, instead if they showed interest in mere learning they were labelled as hyterical and put into mental asylums.
So to consider the question why so few women were engaged in the founding of psychology, it becomes pretty clear. Psychology is mainly considered with Freud and Jung. So the thought of women engaging in it was absurd as at that point of time as women were barred from these segments.
They had very little power or none at all, even if they dared to engage themselves in these men dominating sectors, they were immediately made fun of and it resulted in severe consequences such as debarred from their own society. A few among them such as Virginia Woolf or Alice Walker were able to make their way inside this male wooven tapestry and made themselves heard.