See, originally, copyright was limited to a maximum of 28 years. If you created something, you had 28 years to get all you could out of it, because after that it became public domain. Since those days, copyright terms have been extended numerous times, and each time one company has been leading the charge: Disney.
Each time the copyright on Steamboat Willie is about to run out, Disney loses their shit and lobbies the government to pass another copyright extension law. Although a popular explanation for this is that they’d lose the rights to Mickey Mouse if Steamboat Willie were to become public domain, that’s not the case. Mickey Mouse is actually a trademarked property, and trademarks are perpetual as long as the company continues to use it. (If you haven’t noticed, Disney uses the fuck out of Mickey Mouse.) The simple fact is that Disney still makes lots of money selling DVDs and merchandise relating to Steamboat Willie.
In fact, Duke University compiled a list of all of the films that could have entered the public domain this year if Disney hadn’t argued for the law to be changed in 1976. Movies like On The Waterfront and Seven Samurai, and even the first two books of The Lord of the Rings would be in the public domain now, free for anyone to use and enjoy and remix and learn from. As it stands now, Steamboat Willie remains under copyright until 2023, and even fairly boring things like the very first issue of Sports Illustrated are protected until 2050. You can imagine what that means for movies that came out this year.