It has a lot to do with the era in which it was released. This is during capes and cowls, when none of the characters were even close to realistically drawn/written. The level of maturity and reality in the comics paved the way for the style we read today.
It also used devices that were limited to film at the time. Perspective, flashback, montage, and superimpose being the major ones. It was also a preplanned 12 issue comic, as opposed to a series or a serial. The self-containment of the story, along with the rich world-development done at the end of comics through the supplemental metaworld backstory solidified watchmen as a genre-defining work. In mine and several others' opinion, it shows why comic books are a art in their own right, and that some of the things that the comic does are simply ineffective or simply impossible in film or novels.
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