Some of the financial considerations also deal with cash flow.

For instance, depreciation is without question the lions share of the cost of bus ownership, but that "cost" isn't realized until the day you sell the bus. Kind of like a house. You haven't lost anything until the day you actually sell it. Of course, this is simplified and doesn't take into consideration the loss of use of the funds to purchase the bus, or even the cost of interest paid in the case of a financed purchase.

Maintenance costs, however, are immediate costs and can dramatically influence cash flow.

You can push the "cost" of depreciation off to some future date as it is only a figure on a balance sheet..... but you gotta keep the bus maintained with actual out of pocket expenditures now.