Why is so difficult for some writers to keep a continuous narrative?
Writing, like anything worthwhile, takes time and practice to perfect. Authors may have difficulty with long stretches of narrative for they may need more practice at it.
Stringing a narrative together for a reader is like walking into a room and describing everything in it and what is happening to the people, without putting in the dialogue the person hears. The only people who may have the ability to perfect it quickly are those with hearing disorders. Think of it as pushing the envelope of your writing. Can you, as a person, not speak for any length of time, but still interact with the world?
Other things to keep in mind, if a novel, especially a fiction novel, has no character interaction, then what is the driving force behind the character?
Think of the movie, The Last Man on Earth. In the movie, the main character wonders around a desolate and empty world. In itself, the movie is interesting in concept, but there is only so much you can do with one character and nothing to interact with. Knowing this, the man soon encounters a woman, then another man. Now we have conflict happening as they struggle to understand their placement in the structure of their new world.
Narrative drives the pacing of a work, and dialogue gives life to the prose written by the author. So imagine reading a story with no real interaction? Just a detailed listing of what the person is doing through their life. How interesting would it be?
Generally, a good author will structure their novel with 65-70% narrative. This builds the world, informs the reader of the characters, and is the foundation of what entertains the reader.
A writing friend of mine once sat down and wrote a story that was 98% dialogue. Why that odd number? For he still needed to tag the dialogue, thus creating narrative (even if it is just “he said”). Was the story entertaining? To a point, but not knowing a true physical description of the two characters lessened the enjoyment of the piece, and did not assist in the picture of what happened.
Likewise, I created a flash fiction story called Disposal with no dialogue (498 words and can be found on Mash Stories – http://www.mashstories.com/previously-published/disposal/ – note, the posting is not formatted, so it is better to listen to the audio recording). The story was shortlisted for that quarter, and they called me disturbed.
When you hone your writing art, take time to practice narrative. Try building stories that are descriptive and engaging to the reader, and the only way to do that is by creating narrative.