Luck of the draw. That's not a career in writing. It's started several, and ended several. There are a lot of people who sold spec scripts a couple years ago who are no longer working in the business. Roughly speaking, a spec script's value correlates to how few people have seen it.
Fewer eyes mean fewer passes.
Similar to the aboriginal notion of a photograph taking a bit of one's soul away whenever snapped, a spec script loses its virginal quality when potential buyers are able to actually read it and form a consensus. The brokers of spec scripts thus elevate their value by hyping the unseen pages, since creating a taste before a script hits the market can lead to feeding frenzies. The studios have become these distribution apparatuses. They don't seem to rely on internal development as much as the product driven by a spec script, hyped by very smart agents, who get a bidding war going. Now the material holds them hostage.