[paulat]

And you noted that the maintenance savings are $3 million per year. That buys a LOT of diesel.

$44 million ($47 million - $3 million maintenance) buys a heck of a lot more....

[Ostlandr]

You have to divide the cost difference by the life in years of the buses, in order to compare it to the annual savings in maintenance. (I do this for a living.)
Figure ten years for the life of a bus, that's an annualized increase in ownership cost of $4.4 million. Call it six million with interest.
Subtract the three million in savings, and you're still out three million per year- a losing proposition.
Reduce the cost of the buses, improve their fuel economy, and then they make economic sense.