Copyrights are a deal passed between society and authors to encourage creative production. If they create something new, they can live off it for sometime until their copyright expires. Extending copyrights years after the author's death may serve a corporation or their children that can live on that rent but fails at establishing an incentive for creation.

This video by CGP Grey summaries the problem pretty well (http://www.youtube.com/watch?v=tk862BbjWx4)