To reduce the influence of external factors in the 'value' term. It just makes it easier to measure if one is purposefully factoring out marketing, branding and so forth.

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.