Plot pacing. A novel is written to be read as a whole. An author can have the narrative do whatever is needed as the plot is developed and a conclusion reached by final pages. Extended prologues or epilogues, interludes, sequences from different POVs etc.
A trade paper or hardback starts out as a number of individual issues that are released one at a time over a period of time. The narrative of each issue needs to work both as an individual issue and as part of a larger storyline, particularly when most writers seem to be planning their stories to work better in collected form at this point. Indeed, it's a novelty at this point to find an issue that contains a whole storyline that doesn't tie into a bigger story.
Another factor is the medium. Comics are heavily visual, so tend to have more spectacle. A novel meanwhile can explore characters more efficiently through narrative devices like first person, which can get grating when used extensively in comic form. Extensive first person boxes have been a weak point of the Green Lanterns title so far, and each issue having Jess and Simon introducing themselves and their issues could get annoying when the reading the trade. It's like the early Dark Shadow episodes starting the opening narration with "My name is Victoria Winters". Quite frankly it was a relief when that was dropped. Novels have the luxury of being more wordy, and can afford to be descriptive. Histories and backstories can be alluded to, or a character's impression of another can be conveyed less obtrusively.