What influences certain movies to "get made?"

Screenwriters pour heart and soul into their product.
The market "demands," offerings with mercurial predictability.

The task of the studio appears to answer:

1. What is the market asking for?
2. What screenplay answers this demand?
3. Does this meet my studio agenda?

No individual controls the system, but these three appear to be the Triumvirate of movie making; variously Executive, Judicial and Legislative branches of Hollywood.