That's why I think movies based on his stuff should explore the medium of film the way his works explore the medium of comics. But you really need a very good, intelligent, visionary director for that, whereas Snyder is simply a talented guy who's good at being quite faithful to the source (which is kind of boring if it's too slavish), while in the case of 300, losing some things in the translation and dulling it's edge.
I think before From Hell came out Moore said he was hoping for something like Blade Runner - not a super faithful adaptation of the book it's based on at all, but a ****ing excellent film that captures the themes and spirit of the source. The Alan Moore movie adaptations have been more like all the other movies based on P.K. Dick's work - ranging from tripe to "okay".
Paul Greengrass' Watchmen might have been that kind of great adaptation. Maybe Snyder's will too. V for Vendetta, had it been given to someone like Alfonso Cuaron rather than whoeverthe****, could have been that type of film.
But Moore has said he's not really much of a movie guy anyway. He's just not into them in a big way. That, combined with his experiences with adaptations of his work (especially the fiasco surrounding V for Vendetta), make it really quite understandable that he wan't nothing to do with it. He's just not interested.
It will take one or two REALLY ****ing fantastic films based on his work to change that around.