I think the Zodiac killer must have wanted this to be broken at some point, maybe they overestimated the difficulty though. It's not hard to make an unbreakable cipher (one time pad, or just doing a large number of transformations), seems like using a fairly simple method was an intentional choice. Or maybe they are just insane, but it doesn't make sense to me that you would actually try and securely encrypt something and then submit it to the public, if there is no target audience you might as well just send random characters.