Except if you're not driving the bus at full capacity (i.e. vast majority of the time), you're not saving any labor costs by using the larger vehicle. In fact,

Except if you're not driving the bus at full capacity (i.e. vast majority of the time), you're not saving any labor costs by using the larger vehicle. In fact, the greater maintenance requirements probably cause greater labor costs. Additionally, transit vans don't necessarily need a commercial license to drive, which can reduce labor costs (in part by expanding the availability of the labor pool).