Don Shiach, in his book Movie Classics (Hermes House, Anness Publishing Ltd, 2004), offers these insights:
Strictly speaking film noir is not a genre. It is a body of films that emerged from Hollywood between 1941 and 1958 that shared stylistic and thematic concerns. The term was first used by French critics when they noticed the “blackness” of look and theme common to the American movies released in France after the Liberation. ... [they] remarked on how different these films were from the standard pre-war Hollywood product with its glossy, high-key lighting and upbeat, reassuring message. These film noirs were bleak social documents, turning a disenchanted eye on the contemporary American scene and uncovering a society full of anxieties and divisions.
... However, an opposing view ... points to the wartime and immediate post-war restrictions placed on American film-makers by the US government. Hollywood had to cut down on its use of lighting and sets because resources were needed for the war effort. Thus, the shadowy, dark look of wartime movies came not from the film-makers' gloomy view of the world, but from technological necessity: they could not use extensive lighting, so they used dark sets lit by a few lights or filmed at night. One additional effect of the muted lighting was that it disguised the reuse of old sets again and again because of the restrictions on set-building.
This web page goes further in explaining.