There is a vast area of study on the topic of reading comprehension.
Ability to comprehend text is influenced by readers' skills and their ability to process information. If word recognition is difficult, students use too much of their processing capacity to read individual words, which interferes with their ability to comprehend what is read. There are many reading strategies to improve reading comprehension and inferences, including improving one's vocabulary, critical text analysis (intertextuality, actual events vs. narration of events, etc.) and practicing deep reading …
Reading comprehension and vocabulary are inextricably linked together. The ability to decode or identify and pronounce words is self-evidently important, but knowing what the words mean has a major and direct effect on knowing what any specific passage means while skimming a reading material. It has been shown that students with a smaller vocabulary than other students comprehend less of what they read. It has been suggested that to improve comprehension, improving word groups, complex vocabularies such as homonyms or words that have multiple meanings, and those with figurative meanings like idioms, similes, collocations and metaphors are a good practice …
Some texts, like in philosophy, literature or scientific research, may appear more difficult to read because of the prior knowledge they assume, the tradition from which they come, or the tone, such as criticizing or parodizing.
In short, the difference between Little Red Riding Hood and War and Peace can be broken down into several broad areas:
Some literature (but by no means all) is written in a style that involves long sentences, words less commonly known, and punctuation that can make it difficult to separate out subjects, verbs, objects, and clauses.
In theory, questions and answers on this site are meant to be clearly understood. If something isn't communicated well, it won't fare as well as something that is.
Some literature (again, not all) is written for a specific audience that is more appreciative of the need for a higher level of reading comprehension.
Having said all of that, I would be careful of using a distinction like regular English and literary English. Some so-called regular English can be quite difficult to understand. Meanwhile, some so-called literary English can be quite easy to understand. It really depends on what specific examples you are thinking of.