Its because Watchmen is a one-off story with a beginning, middle and end rather than a 40 year old serial to draw influences from, and the story was specifically written in a way that took advantage of the comic book format.
The fact that Watchmen is a one-off story doesn't hold water. Almost everything that's been adapted into a movie has been a one-off story. Think of any book or play that's been made into a movie.
As for your second point, honestly I think the "comic book format" argument is way overplayed. What can comic books
really do that movies can't? They have panels as a visual medium... that's pretty much it. Use of panels was a nice little gimmick for Fearful Symmetry, but it didn't add much to the story, themes, message, or any of that. The story is what's being adapted, not the comic book.
If anything, I'd say regular books are more difficult to adapt to movies than comic books. They're much richer in terms of detail, they make extensive use of the narrator's voice/perceptions as a device (
One Flew Over the Cuckoo's Nest comes to mind--another example of an "impossible" adaptation). They make use of poetic imagery to make the reader imagine things rather than just showing or telling, in long descriptions of scenery, or a person's appearance, etc. Relatedly, books can take pages upon pages to describe events that in real time (or movie-time) would pass by in a few seconds, to give insights into characters' minds, thought processes, or what have you.
Comic books by comparison are inherently cinematic, because they lack these things, and are explicit in what is shown, said, and presented.