Many factors have contributed to the shrinking public domain. Over the course of U.S. Copyright Law, the length of copyright protection has been extended many times. The Sonny Bono Copyright Term Extension Act of 1998 extended copyright to:
- life of the author plus 70 years, or
- 95 years for a work for hire.
Another contributing factor was the change in the use of the copyright symbol (¬©) in 1976. Prior to then, the symbol indicated copyright ownership, and its absence indicated that the work was in the public domain. After 1976, the symbol was not required to indicate copyright ownership and all materials were automatically under copyright.
A useful chart to clarify the complex nature of defining the public domain can be found at the web site of Cornell University, originally created by Peter Hirtle.
Creative Commons provides licensing options for authors to assign various levels of copyright, including open access with attribution.
One effort to reverse the continuous extension of copyright duration was the case known as Eldred v. Ashcroft. While this attempt to reverse the Sonny Bono Copyright Extension Act of 1998 was unsuccessful, it drew public attention to this issue. While the ruling is covered in the above link, background information is available at the Berkman Center for Internet and Society.