The answer comes from the kinds of space these trailers decide to show. While the bulk of the drama and action of The Crazies seems limited to smaller-budgeted horror setups—stuck in a house, stuck in a car wash, small jails, small hospital, small town—the trailer is cleverly structured in its imagery to suggest a subtle, gradual expansion of the space and scope of the picture. Early shots of the town empty not as a final revelation but as part of the evolution of a horrible situation, the unexpected intrusion of the military as shown by helicopter shots and army vehicles, the generic title cards "this is only the beginning," the steady suggestion of the lead characters fleeing the town in vehicles at the trailer's end, and, finally, the last third's use of the song "Mad World" by Michael Andrews all successfully work in conjunction to suggest that what we are seeing—mostly images of a low-mid budget and very generic horror film—are only a small segment of a larger and more mysterious story.
Space—material, conceptual, imaginative—is opened up through these techniques and helps The Crazies seem bigger and potentially more surprising than it really is, as if the bulk of the movie occurs after what we are seeing from the trailer.
The lack of appeal of Paul Greengrass's new film from the trailer is due to the opposite approach. Greengrass's iconic docu-impressionism, with its rapid cutting, lack of clear composition, and frantic hand held camera movement, is intrinsically a difficult style from which to conjure a sense of differentiated space. For the Bourne movies this wasn't a problem, as Bourne's globe-trotting persona and the movies’ use of multiple, varied action sequences made for trailers whose flurry of images went from locale to locale, one kind of action (a flurry of hands) to another (spinning cars), suggesting enough variety that one's imagination could work in the space in-between.
Not so with the Baghdad-set Green Zone, whose trailer suffers from a muddy and stultifying sameness, making it difficult to judge where any scene is set, how the movie might progress, what kind of visual elements might make up the story. This lack of imaginative space shuts down our ability to suppose that the rest of the movie might even feature more footage. In a way, the Green Zone trailer resembles a fake trailer for a feature film that doesn't exist, because it fails to suggest anything outside of the footage it itself contains.
Compare the trailers for The Crazies and Green Zone. The little horror film cost $12 million to make, the other, an action epic directed by Paul Greengrass and starring Matt Damon, probably 10x that. Yet, from the trailers, it is The Crazies that looks better, bigger, more ambitious. Why is that?