It used to be hard to copy. This greatly limited the effect of copying. For example in the 18th and 19th centuries, books made almost all of their money on the first print run. By the time a copier could get a copy out, there wasn't much money left to be made from the book, and so it was hard for the copier to recoup his copying costs.
Justice Breyer, back when he was a law professor, not a Supreme Court justice, wrote an interesting article about this, arguing that copyright throughout most of its history was in fact unnecessary because of the delay and costs of copying.
From the latter half of the 20th century onward, copying technology has gotten cheaper and faster. That has shifted the balance. The copier can now, especially with digital goods, get their copy out to the market fast enough to reduce or eliminate the sales of the original.
Justice Breyer, back when he was a law professor, not a Supreme Court justice, wrote an interesting article about this, arguing that copyright throughout most of its history was in fact unnecessary because of the delay and costs of copying.
From the latter half of the 20th century onward, copying technology has gotten cheaper and faster. That has shifted the balance. The copier can now, especially with digital goods, get their copy out to the market fast enough to reduce or eliminate the sales of the original.