I'm not sure "driving force" is really accurate. The main impetus for the 1976 Act, which changed the term from 28 years with the possibility of one 28 year renewal to life + 50 was to allow the United States to join the Berne Convention. Berne required a minimum of life + 50.

The extension to life + 70, or 95 years after publication for corporate works, was in the 1998 Act. Disney did lobby for that, but the big argument for it was that was to match EU copyright terms, which went to life + 70 under the "Directive harmonising the term of protection of copyright and certain related rights" in 1993.