I think John Wagner should do it.
Why he hasn't is beyond me.
Probably because he's a comics-writer not a screen-writer, two completely different diciplines. The amount of work required writing and re-writing a screenplay is a full time job even for a pro screen-writer and he'd need to reduce his comics work greatly, or completely, for a time in order to do so. He could do it in collaboration with a screenwriter, it would also be less likely for a non-screenwriter solely to have their screenplays picked up.
There's a huge difference between how a comic is constructed and a screenplay. Different Pacing, plotting, character development, scene construction and a different type of dialogue too because it is written to be said not just to appear in a bubble on a page in a frame beside a anthropomorphic squiggle of lines. Not saying he couldn't do it and do it very well, but it's a bit like deciding to be plumber when you've just trained to be an electrician.
