To answer that I have to tell you where comic books come from. I'll go way back into the nineteenth century when they had cheap fiction books called Penny Dreadfuls. Often these were serialized novels with characters like Varney the Vampire and Sweeney Todd, the Demon Barber of Fleet Street. Over time these became magazines which were printed on cheap paper and were called the Pulps. Tarzan of the Apes and Zorro who both date from around the time of World War I were two heroes who emerged from pulps -- onto the silver screen.
Now comic strips were emerging too, but the editors wanted gag strips which people didn't have to follow day to day for a long time. Still, sequels and series featuring one hero sold. Thus in 1929 Tarzan was adapted into a lavishly illustrated newspaper adaptation by Hal Foster which was so popular people suddenly realized in was a comic strip, and as the Return of Tarzan was adapted by Rex Maxon a Newspaper editor named John Flint Dille bought a story called "Armageddon 2419" and adapted it into Buck Rogers. Other heroes joined them in the pulps-- Conan the Barbarian, Operator Number 5, the Shadow, Doc Savage the Spider and 2 versions of the Bat. A teenager named Jerry Siegel, inspired by an SF novel called Gladiator by Philip Wylie (whose The Savage Gentleman had been adapted into Doc Savage without credit and whose When Worlds Collide had inspired the opening page of the Flash Gordon Comic Strip) created a character he named Superman. Superman didn't sell to the comic strips though a character named the Phantom did. One thing that happened however was that when comic book publishers, who were doing reprints of comic strips, decided to compete with original material, he and Joe Schuster sold their character there and the superhero as we know it was born.
Meanwhile Hollywood was making a lot of money off cheap fiction. The Shadow was a successful serial starring Kirk Allyn. Flash Gordon and Buck Rogers made something of a star out of a swimmer named Buster Crabbe, who had been beaten by Johnny Weismuller as a swimmer and was again at the box office by Weismuller Tarzan. Superman became a serial starring Allyn before it became a TV series starring George Reeves in the fifties (I used to watch it when I was very little). That is where comic book movies came from. They are just adaptations of cheap fiction.
They are popular because they are sold. As a kid I was told "of course you like Western Movies". I knew very well I didn't but it was like nobody cared. I was American so I wanted to watch Cowboy movies thank you. As I reached my teen years SF Fantasy and Comics broke out big time and I had fun for a while till with Star Wars I realized nobody wanted to make SMART entertainment for common people, they just wanted to make STUPID entertainment for common people. And yes, that is how they are doing it. Most Anime fans will pay lip service to the X Men movies but by NO means ALL and some won't. And they aren't the only kids I meet who feel about comic book movies the way I used to feel about Cowboys back in the sixties. I will see John Carter when it comes out, which is SF (based on a 1914 Edgar Rice Burroughs Novel) but which if it's made right will start out as a western with supernatural undertones. Yes I've mellowed and the director sounds like a smart man as well. Anyhow, that is what comic book movies are, and that John Carter movie is a pulp movie of the sort that comic book movies are derived from.