I'm a writer, and I have sold scripts, and have a movie in post now I'm hoping will come out soon. And believe me, if I knew the answer to that question I'd be rich.
I do know some things. As in publishing and music, most of the studios are owned by people for whom the bottom line is money, rather than people who geninely love movies. They buy scripts that fit into fads, whatever the current big sellers are, like movies based on comic books and shoddy remakes. They won't buy a film unless they feel it's guaranteed to make a few bucks.
You can no longer sell a script to most studios unless you go through an agent. No one will even look at your work. And, in a way, that's a good thing. This doesn't mean I LIKE that setup, but so many people THINK they can write a good script, when in fact they can't. Writing a script is far harder than writing a book. You have much less time to accomplish everything you need to accomplish.
Let me tell you something a high end producer once told me about how he looks at scripts. Before he even reads it, the first thing he does is to flip through the pages, just looking at the form of the script, to discover if the balance of action to dialogue is in good shape. And if it's not a good balance, he won't even bother reading the script. He knows it won't be good.
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