Julian Sanchez has a theory:
Insanely long copyright terms are how the culture industries avoid competing with their own back catalogs.
Imagine that we still had a copyright term that maxed out at 28 years, the regime the first Americans lived under. The shorter term wouldn’t in itself have much effect on output or incentives to create. But it would mean that, today, every book, song, image, and movie produced before 1984 was freely available to anyone with an Internet connection. Under those conditions, would we be anywhere near as willing to pay a premium for the latest release? In some cases, no doubt. But when the baseline is that we already have free, completely legal access to every great album, film, or novel produced before the mid-80s—more than any human being could realistically watch, read, or listen to in a lifetime—I wouldn’t be surprised if our consumption patterns became a good deal less neophilic, or at the very least, prices on new releases had to drop substantially to remain competitive.