I believe many early animated films were based on print comics because many comics were already popular and have established characters that transferring them to a new medium was a safe way to ensure people will come to see your new film. Another reason I believe film were based on comics are that comics as a visual medium made it easier to draw the keyframes that would be needed to tell the films story. If you think in modern day, a comic strip works similarly to a storyboard where the pictures layout the framework of how the story will playout, and the role of animation is to create the motion from one panel to the next.