I don't have the hard data in front of me (I hate researching strips, personally; there are too many places the owner MIGHT have filed it), but the inconsistency question rarely has a good/interesting answer. For the most part, comics especially were only considered an ephemeral product to bring people in to see something else (stories, advertising, or whatever) and wouldn't have much interest once they were "out of date."
Since it cost money to file the copyright registration form, and most people would use a lawyer to manage the form, it just wasn't worth the money. However, once markets opened up for licensing, the publishers or authors usually decided to take some of their earnings and protect what they could.