The simplest answer I've heard as to why the public domain stops in the 1920s is this: Mickey Mouse. The Disney Corp. doesn't want to lose control of Mickey, whose first movie was in 1928, though I'm told that early versions of him appeared in the background of other cartoons a few years earlier. Every time there has been discussion of moving public domain forward, Disney sends in lawyers to stop it, and they'll keep sending in lawyers because it's cheaper than losing the rights to their founding character.