Comics, especially the good ones, use so many different ways to communicate message, story, themes, the gist of whatever, and are sometimes more complex in that respect than any other media, which makes it hard to translate into moving imagery and sound.
Examples: Art, art styles, art styles within art styles, impossible characters, super duper mega effects that are unteanslatable because of the cost of ink vs the cost of weta working their asses off for a yearm, rapid cuts in viewers perspective, coloring, line work, page layout, typography, up to commonly three or four streams of text (dialogue, yellow boxes, thoughts and onomatopoetry (Bam!)), insertion of prose and documentation, characters retelling the entire story to another character, different artists, comics within comics (watchmen and the pirate), multiple main characters (team books), time jumps (and travel), complete disregard of or just lack of necessity to adhere to the aristotelian drama (act 1, 2 and three) and much more.
It’d be lik Walt Disney and David Lynch plus ILM and WEtA directing a half cartoon but nightmarishly surreal underground simulation of a roller coaster in space told backwards with eleven different languages spoken with and without subtitles covering the action…
Does the point come across? Complexity.
Ask Alan Moore.