In this case, could it be at least in part a function of the likelihood that people will listen to and/or perform a piece of music after the copyright on that work has expired? Assuming such a thing is possible, of course.
It's fine to expect the sales of a work to go up if someone performs it well for a large enough audience, but is that measuring the value of the work, the performance, or the exposure? And basing the function on a period after copyright expires reduces the influence of stakeholders in the work who might pay for exposure.
http://www.youtube.com/results?search_query=cover+teenage+dr...
Check out the blues available on archive.org sometime. It's pretty amazing, in my opinion.