Over at his excellent blog, Jez Freedman writes about the trend towards people in film and TV looking to other storytelling media to find new talent. I was going to comment on his blog, but my reply got too long to offer there so you're getting here it instead.
I think part of theatre's attraction for script editors and development executives in search of new talent is getting out of the office. Reading a mediocre screenplay is just dull. At least if you go see a dull play you get out of the office, maybe have a drink or two, a bite to eat aftewards. Plus there's acting, set design and the costumes to admire, direction to appreciate - it's an interactive experience.
Theatre also offers the intrepid exec that thrill of discovery. Pluck a playwright out of nowhere [at least, nowhere in terms of the TV/film world] and you get kudos for finding new voices. If they succeed, you can bask in the reflected glory of their subsequent acclaim and awards. [I still take pride in having helped quite a few comic creators on the path to long, illustrious careers.]
I suspect there might be an element of novelty, too. If you're reading thousands of pages of screenplay every week in search of new voices, your eyes cry out for something that isn't presented in Courier 12 pt screenplay format. Along comes a script for a play, totally different format, different font. I imagine it'd be a welcome change of pace.
Plus scripts for plays are largely comprised of dialogue. Even if you don't see the play on stage, you can swiftly assess from the script whether the writer has a knack for dialogue - can they make each character seem different? In that sense there is no hiding place, no poetic scene description to disguise having a tin ear for dialogue.
[Of course, that raises the question of how you can accurately assess a playwright's ability to tell stories purely through visuals. But let's be honest, how much storytelling in TV drama actually happens through visuals? I'm sure you could name a few shows that - due to time and budget constraints - are more akin to radio with pictures. Truly visual storytelling gets reserved for high end TV drama, alas.]
But there's a truth that doesn't get talked about with emerging playwrights. Their work will have been workshopped by actors, directors and dramaturgs, a single play going through months [even years] of readings and rehearsals. [There's nothing to stop screenwriters doing that too, but it isn't easy to fund!] How much of the performed script is actually written by the writer?
Just as celebrated new authors sometimes suffer from different second novel syndrome, so there are plenty of playwrights who can't reproduce their early genius once shorn of the support mechanisms that helped them achieve that breakdown. Hot new playwrights, authors and radio dramatists can make great screenwriters, but most need to learn a set of new craft skills first.
I would argue that it's not when you come from, it's what you do on arrival that counts as a screenwriter. In other words, you're only as good as your lastest draft. Onwards!