My understanding is that it is to a great extent voluntary; almost entirely voluntary. Marching band members take a class that is - I believe - one single credit; as such, it is not anywhere close to being commensurate to the time they put in to be part of the band. The 'pep bands' at the other sports/events are done on a volunteer basis, drawn from the marching band, with 'seniority' in effect for 'in demand' events, like men's basketball, etc.
So what you're asking is "Why didn't those college students who volunteered a couple hours of their weekend to play at the women's basketball then follow that up by also volunteering an additional couple hours of their weekend to play at the hockey game?"