It could be that the plot or images in the comic are so engaging that they keep your brain stimulated. Also, if you read it right before bed, the light from the comic might disrupt your body's natural sleep cues.
There could be several reasons. Maybe the comic has a tense or thrilling storyline that keeps you on the edge of your seat. Or perhaps you're in an uncomfortable position while reading it. Sometimes, if you're already stressed or worried, even a comic can prevent sleep.