Why so popular? Because the storyboarding and visuals are already sketched out by the original issues of the comics themselves.
Adapting a novel requires an imaginitive F/X team to create the F/X from mere text descriptions of the scenes and items to be depicted. Having existing pictures makes it cheap and easy to skip that creativity in the process.
There is also the fact that an awful lot of movies adapted from novels just tank at the box office because they don't express a vision that the readers of those novels had in mind. Even short stories tank. Take, for example, "Enemy Mine." It was a great short story, but kind of sucked as a movie.
I'd like to believe that a better job could be done by a competent team with a good budget, but then along comes something like "Ender's Game", which was so bad I gave up on watching it less than half an hour in. Yet I'd devotedly read the entire series of novels set in that world in my high school days, and enjoyed them thoroughly.
I've often wished they'd get around to adapting some of C. J. Cherryh's universe to a movie format, but I fear they'd butcher her excellent writing and characterization and leave us with yet another F/X fest that tanks at the box office and loses all the appeal of the novels.