Because band parents are more committed to help than athlete parents or boosters. The major reason bands get the concessions is that they have the ability to man the concessions through volunteers. As someone else in the thread said, people just don't want to volunteer for stuff anymore. But band parents at most schools are still willing to do so. I'm sure athletic departments would rather keep the money but would not be able to get enough volunteers to do so. And if the band is successfully running the concession stand, I doubt people want to risk changing it up.