I would encourage you to go read this post at The Mary Sue about why Doctor Who has not had a single female writer since 2008.

If a show has zero female writers, it’s not because there are somehow zero female writers who are qualified to do that job. In the Guardian article [that inspired the Mary Sue post written by] Jenny Colgan, who wrote a Doctor Who tie-in novel, [she] points out just a few female writers with a “proven track record” in sci-fi, among them Margaret Atwood, J.K. Rowling, Ursula le Guin, and Suzanne Collins...the TV and film industries, more than just having a long history of being male-dominated, were built upon the idea that men hold leadership positions, whether it’s being a director or a writer or a studio exec. With few exceptions, that’s how things were in the formative years of the industry, and that’s how they’ve continued to be, even though few(er) people think (or would admit to thinking, at least) that women can’t do the job.

Television writing is still a male-dominated job. (For proof of that, go read Lauren Weedman's great piece about her time writing for The Daily Show.) The only way that's going to change is if people demand to see a change. Because television writing is such a non-visible role, though, the demand is low. And because men write for television, we're less likely to see strong female characters on television. It's a cycle that's only going to stop if we shine some light on the process.