For more about the cheese, including its ties to Parma, try here:
http://www.parmigiano-reggiano.it/home.cfm

Given that it's the "parmigiano" that gives English its "parmesan", one might ask with equal validity, "why was the 'g' dropped and an 's' used in its stead?" Perhaps the answer to both questions is that the changes were made to underscore the gulf between parmesan cheese and parmigiano-reggiano, the one being no more than a vaguely sound-alike knock-off of the other.