Again, because "volunteers" is such a great name for a sports team compared to "bulldogs, eagles, falcons, gators, tigers, wildcats". etc.
It's based on the state's nickname; one it earned due to a huge/massive number of people enlisting during the war of 1812 (and i think either the Mexican-American War or the Spanish American war)
Not saying through that any one of these names are better than any others.