I would think it is only because they are bad.
No plot to speak of, rubber stamp art, or reality.
I like the Graphic Novel format, just not what is being produced.
The lead characters seem to lack insight, intelligence, or a sense of humor.
Settings are not developed, and storylines go nowhere.
Some write to give the reader a mental picture, and most do not get it, they cannot stay focused for long enough and lack the ability to make their own mental pictures.
Comics main function is images, with little story, and that not complicated.
In that, they were the cultural predecessor of TV. Dialing for Dollars, or the Price is Right?
Your big books without pictures are read by few, readers have always been a smalll group.
The exploitation of books is recent, since the fifties, when standards were lowered to increase readership, ending in Stephen King, writing a comic book as a movie script, in print.
The function of writing as social commentary has been lost. Writing, the tool that allowed thought to be preserved from generation to generation, is now just a media outlet.
It is, but does not have to be. The Graphic Novel holds great promise, and Dicken's works were illustrated with copper plate engravings, for images can add to a story.
It is a medium halfway between books and film, and can reach a readership much broader than print alone. Now if it only had wriers, artists, it could and will become something.
I think the next step is digital delivery, some animation, spoken lines, for paperless delivery.
It is an art form in search of art.
My own book is being rendered as a Graphic Novel, working out each scene as a storyboard.
Read this book and make a film, just does not work. My illustrators say draw me a picture. The book becomes film script, then storyboards, for the production crew needs explicit direction. These are the best of modern storytellers.
Comics do sell in the billions, and a better connection between story and reader is needed.
This life problem has been met before, by Jane Austin, The Sisters Bronte, Mary Shelly, and they did see a void and fill it.
The hip consumers of your time are going for flash over subtance.
Only sixteen, and old. Older yet soon, as the hopeful of your time sink into the fog of day to day living, leaving you with the world of books.
Jane Austin faced a world of another play by Shakespeare, badly played, another Italian Opera, or the low and vulgar humor of traveling players in village taverns, places no decent woman would go, but the lines were repeated by the servants.
She saw the need for something between, broadly entertaining, yet containing higher social values. Something new, The Novel.
You are looking upon the same problem, in another age.