Electricity is not magic. There is exhaust at the power plant where the electricity is produced unless it is nuclear, hydroelectric, solar or wind. To power an expanded monorail system would require a quantity of solar panels that would make the whole property a giant eyesore (and likely require cutting down a lot of trees to fit them). Diesel busses (especially modern ones) are very efficient.
If monorails were that much more energy efficient and put out that much less emissions (when considering the whole energy chain) than dirty diesel busses, cities would be building monorail systems instead of having busses. This would be especially true in cities like San Francisco.