It goes back to the primary purpose of copyright law -- protecting the interests of the person who wrote the stuff in the first place. I think the general thing was 75 years after the death of the original copyright holder (to protect the rights of the holder's family too). They only really protect the
public interest indirectly in that they ecourage folks to generate intellectual property and sell it.
These days it more or less gets extended everytime Walt Disney's (the man) copyrights come up for expiration, Disney (the Co.) has been successfull in extending them so far.
One interesting bit of trivia here is that
Peter Pan's original
copyright is actually held by a
children's hospital in England, so it basically won't expire as long as the hospital is around.