Genie, take a look at the "Sonny Bono Copyright Term Extension Act of 1998," passed after heavy lobbying by certain members of the entertainment and publishing industries.

The way I heard it, back in the early 1990's certain large US holders of valuable old copyrights went to Europe and lobbied to get the term of copyright extended over there. Then they came back and lobbied Congress, complaining that US works didn't get the benefit of a longer copyright term in Europe that European works did, because pursuant to an international IP treaty the longer term would only apply if the US extended the term of copyrights in the US. Congress listened, passed the Copyright Term Extension Act and now nothing new has dropped into the public domain due to copyright expiration since that time. Works published in 1924 whose copyrights would have expired in 1999 instead won't expire until 2020.

This is not the first time the term of copyright has been extended, and there were some major changes in the mid-70's and early 80's due to the abovementioned IP treaty. I suggest keeping a sharp look out for another move to extend the copyright term sometime around 2018 or so. :-)   

The funny part is that back in the 19th century, the US didn't respect other countries' copyrights at all. When Charles Dickens came to visit, he was pleased to meet his many fans, but said that he wasn't earning a dime on a single US copy of his works-- at that time, US copyright law didn't protect foreign authors.