Under current law in most of the world, copyright lasts for 50-90 years after the death of the creator. In the U.S. it lasts instead for 95-120 years after the work is created. That’s a long time. But it didn’t used to be so long.
When copyright law (as we know it) was established in the 18th century, the term in the U.S. was only 14 years, with a 14-year extension available if the creator was still alive. This was a temporary monopoly on the work, and intended for living people still creating new works, which was spelled out in the U.S. Constitution. In the 19th century the term was doubled to a 28-year initial term, and then the extension was doubled to 28 years too: a total of 56 years.
This lasted until the 1970s, when – under pressure from the entertainment industry, which had a lot of early-20th-century copyrights about to run out – the U.S. Congress extended those 56-year copyrights to 75 years, applying that retroactively to any copyrights still in force (disregarding the clear intent of the Constitution). Twenty years later, they added another 20 years, with an additional 25 years if the work happened to be corporate-owned, another obvious gift to Big Media.
(The length of the terms and the history are different overseas, but the general trend is the same: longer and longer copyright terms.)
The end result of this is that two human generations of works that were supposed to enter the public domain, have instead stayed in the copyright vault. No additional works have been freed from copyright limitations in a third of a century; the public domain calendar has been stuck, limited to “1922 and before” the whole time, and will stay there until 2018… unless it’s extended again.