Here is a rough summary of how the terms have evolved under U.S. law (the details of which are nicely summarized here: http://ipwatchdog.com/2011/05/27/copyrights-last-for-a-limit...):
1790: maps, charts, and books - 14-year term
1831: 28-year term with option to renew for another 14 years (musical compositions, prints, dramatic compositions, photographs, works of art and music added to list of protected works in 1800s)
1909 (a major act): 28-year term from date of publication, renewable for another 28 years (applied to "published" works only, with state laws governing unpublished works) (motion pictures added to list of protectible works)
1976 (another major act): newly published works (life of author plus 50 years); works copyrighted before 1978 (term increased from 28 to 47 years, for 75 years total with renewal) (applies to all works, whether or not published, once in a fixed medium of expression; state laws preempted and no longer valid; computer programs also protected by the 1980s)
1998 (Copyright Term Extension - "Sonny Bono Act"): works created after 1978 (life of author plus 70 years, with joint works tied to life of longest living author, and with works-for-hire, i.e., corporate authorship, getting the shorter of a 120-year term from date of creation or a 95-year term from date of publication); works created before 1978 (total term increased from 75 to 95 years)
The rough summary above is just that and many nuances exist. If this all makes your head spin, there is a nifty "public domain calculator" to assist you, found here: http://www.publicdomainsherpa.com/calculator.html
The trend is pretty obvious: wildly long terms for copyright are in fact a relatively recent phenomenon, as are legislative efforts to go back to early works and give them ex post facto term extensions. It didn't always used to be this way. The focus seems to have moved away from protecting primarily an author's rights to a creative work during his lifetime to protecting a "franchise," often corporate, that lasts well beyond the lives of those around when a work is created. These are policy decisions, of course, albeit (and sadly) much influenced by lobbying today.