Watchmen the graphic novel was, despite its novel-ness, episodic. Like TV shows. Like a serial comic book.

Episodic things don’t tend to translate well into feature-length films. Producers have two options: new content based on that universe (a la the various Star Trek films), or a heavily cut summary of some major points (a la Batman Begins), but even the new Batman films didn’t try to cram everything into one film, they left room for other niftiness in sequels.