Zack Snyder had to condense a classic 12-issue series into a feature film, which meant that there were necessary cuts to the source material, even if some have opined that the movie suffered for sticking too closely to the comics, which were written by Alan Moore and published from September 1986 – October 1987.
Perhaps a TV show could do more justice to the source material by hitting more of the big moments and expanding on characters in ways that are not possible in a movie.
The 2009 movie undoubtedly introduced many people who had never heard of the Watchmen comics to the universe, but it wasn’t quite the box office hit that comic-based movies are nowadays.