One major factory is the significant gender inequity that exists in the recruitment, promotion and retention of female faculty and staff scientists.
In the 1980s when I was a medical resident at the University of Chicago, I was among the largest class of females the program ever had. My female peers and I faced many challenges, including overcrowded on-call rooms. They simple weren’t ready yet. But, we were.
Read more here: https://blogs.scientificamerican.com/voices/why-arent-there-more-women-leaders-in-science/