Many early films were based of off adaptions for comics because they already had a pre-existing fanbase and an easy story to work with. Comics are similar to animation as they are both defined as different types of image sequences. There was not a lot of pre-production work that had to be done because the comic artists and creator had already done this for them. Additionally, it was generally cheaper for them to work with something already existing instead of making and developing characters for animation which was not a common thing to do back then.