This was a backlash that developed after the reception that "poetic realist" films had achieved over the previous several years, which spawned a mini-proliferation of dark films in all of their variants (policiers, alienated colonialists aka "poetic realism," spy/exotic).
The films did not begin with a big fanfare in France, but directors and writers kept making them anyway. Harry Baur became a star in various variants of these films in the first half of the thirties; Gabin emerges in the mid-thirties and gives poetic realism a poster boy, which is why that strain of French noir eclipsed all the others and is the one still given the lion's share of attention.
Speaking in general, film can't expand its language as rapidly as music--it is not immediate enough of an art form for that. As for noir, since it was mostly a manipulation of certain plot elements, an intensification of character conflicts, and a use of lighting/editing/camera angles to accentuate that, it was not as autonomous a form and had more limits on how it could evolve. It was also constrained and shaped by the studio system in America, which had opened up its approach during WWII to accommodate stories that attempted to address and expiate homefront guilt and dread; once those gates had been opened, subsequent events permitted an explosion of such films in the second half of the 40s built around anxiety, uncertainty, and a temporary fixation on moral ambiguity.
In France, the Occupation limited and controlled production, forcing the directors/writers who did not escape to create a subtextual subgenre ("provincial gothic"), which proved to be protean and adaptable, continuing on after the war in various hybrid guises.
In Germany, the post-war conditions produced a short period of searching self-examination in what is called the "rubble" film, or zeitfilm, which despite significant success in exploring dark themes, ran its course in only a few years.
We'll be doing a series in March about these German films...more details about that very soon.