Why do so many movies fail the Bechdel Test?

For a movie to pass The Bechdel Test, it must:

1. include at least two women...
2. who have at least one conversation...
3. about something other than a man or men.

This seems like a rather low bar and yet about half of all movies fail this test (http://bechdeltest.com/statistics/).

This question was orriginally posted on Quora; here is my answer to it:
Interestingly enough, most Horror movies (for which the primary target audience is young women) pass the Bechdel Test. Also, the movie Conan The Barbarian 3D passes the Bechdel Test, barely, because I changed one of the villains to a woman (Marique, played by Rose McGowan) and gave her a scene with the leading lady in which they talk about resurrecting a dead witch. Go figure.

I would argue that the Bechdel Test reflects what movies get greenlit and what audiences have traditionally shown up to see, not a flaw in the screenwriting or even the development/rewrite process.

In my own crreer, I've written several unproduced screenplays in which the lead character, the main supporting characters and the villain, are women. An example is Blackwell, a thriller based on a real event in the life of Nellie Bly, one of the first female investigative journalists. The movie, set in 1888, follows Nellie as she fakes insanity in order to go undercover as an inmate at Blackwell's Island, an impenetrable women's asylum. However, once inside she discovers that it is nearly impossible to get out.

Although A-list actresses such as Ellen Page and Charlize Theron have shown interest in the script, the fact that it has a "female lead" makes it difficult for my producers to set up at a studio.

The factors that make a large number of films that make it to the screen fail the Bechdel Test are:
1. Most Hollywood films have a male lead character. Statistically speaking (and Hollywood loves statistics) movies with female leads open significantly worse at the box office than movies with male leads.

2. Most (primary) villains are male. Although fabulous female villains to appear in movies, the majority of villains (people who are powerful, sadistic, violent, and otherwise bad) tend to be male. Perhaps men are generally thought to be more threatening.

3. "Character actors" tend to be male. The quirky supporting character with a funny nose, strange voice, or flabby belly, are typically dudes. Audiences (again, statistically speaking) like their actresses young and pretty. There isn't a "Steve Buscemi" among female character actors.
4. "Best Friend" or sidekick characters are usually men. Female leads sometimes have male "best friend" characters (often gay,) but Male Leads almost always have male "best friend" characters.

5. The most popular "hot female stars" (i.e. the one's on the cover of magazines) are in their twenties to thirties and most often play the "love interest." The most popular "hot male stars" (the ones on the cover of magazines) tend to be in their thirties and forties, and play lead characters.

6. Items 1 through 5 mean that most scenes between two characters in a movie will have at least one man.

7. Since protagonists and antagonists in movies are usually men, and a scene between two women is likely to be "about" the protagonist or antagonist, as they are the major characters in the story, conversations between two women are likely to be about a man or men.

8. Movies made for women tend to be Romantic Comedies or Romantic Dramas. Thus, the subject of conversations is most often tend to be about romance and relationships, i.e. men.

So it nothing to do with the screenwriting process, and everything to do with which screenplays are chosen to get made.