There aren't more lady songwriters for the same reason that there aren't more lady doctors or lady accountants or lady lawyers; not enough women have the time for careers.
“I was married awfully young and I felt trapped. My wife had been divorced and all the time we were married we were out of the Church. It wasn't until we were divorced that we became good Catholics again.”
“It is difficult to get men to pick up a female author. Women will read men, but men won't read women.”