From the Copyright Term Extension Act wiki entry:
https://en.wikipedia.org/wiki/Copyright_Term_Extension_ActSince 1990, The Walt Disney Company had lobbied for copyright extension.[13][14] The legislation delayed the entry into the public domain of the earliest Mickey Mouse movies, leading detractors to the nickname "The Mickey Mouse Protection Act".[4]
In addition to Disney, California congresswoman Mary Bono (Sonny Bono's widow and Congressional successor), and the estate of composer George Gershwin supported the act. Mary Bono, speaking on the floor of the United States House of Representatives, said:
Actually, Sonny wanted the term of copyright protection to last forever. I am informed by staff that such a change would violate the Constitution. ... As you know, there is also [then-MPAA president] Jack Valenti's proposal for term to last forever less one day. Perhaps the Committee may look at that next Congress.[15]
Other parties that lobbied in favor of the Bono Act were Time Warner, Universal, Viacom, the major professional sports leagues (NFL, NBA, NHL, MLB), and the family of slain singer Selena Quintanilla-Pérez.[13][14]
The two main reasons for the extension:
1. to protect U.S. works in foreign countries, and thus ensure a healthy balance of trade;
2. life expectancy rates rose, which means creators should be allowed to receive continued benefits from their work.